Monday, October 31, 2005

Pond design to deter herons catching fish

If you are thinking of building a pond, one point you should consider is the possibility of herons catching your beautiful fish. It is heart rending to discover the remains of a young koi or even two or three small fish have mysteriously disappeared overnight from your pond. So at the design stage you should consider making your pond awkward for herons to fish at.

Many people consider a first step or shelf in the pond as essential for planting marginal plants. This first step is usually 9 to 12 inches deep, and it is precisely this sort of depth that herons like to stand in because it brings their head and beak within easy striking distance of the water.

They fly down and alight at the side of the pond, then gently step down onto the shallow shelf where they wait patiently until your fish swim lazily by, unaware of the danger, and then the heron strikes. Breakfast!

So when designing your pond:-

If you really want a shelf, make sure you occupy ALL of it with marginal plant pots to make it awkward for a heron to stand between.

Or only have a shelf for a shorter distance around the pond, and give plenty of depth for the fish to go deep around that section of the pond also.

Netting is usually the best deterrent after the pond is complete, although there are various gadgets available which some people say are effective, e.g. trip wires, water guns, infra-red detectors that make high-pitched noises to mention but a few. The trouble with these gadgets is that if they don't find work it can be an expensive and frustrating way to find out.

Alternatively a real dog or cat visible in the garden will make herons cautious of approaching, but usually a heron fishes very early in the morning before pets are out in the garden, usually shortly after dawn at around 4 to 6am.

Plastic dummy herons are also said to fool passing herons into thinking that "patch" is already occupied by another heron.

Also important is to give your fish a hiding place. If they feel something is wrong they will go deep, so give them something to use, a short section of wide diameter drain pipe, or some tall reeds to hide below, or even some kind of out-cropping or overhanging shelf (but again not something a heron could stand on).

The best defence is simply not to make it easy for the heron in the first instance!

Sunday, October 30, 2005

Ziks DIY Venturi Design

Soon after I had built my early design of Skippy style bio-filter and discussed it on one of the forums I enjoy, I received some advice from a guy named Zik on the forum. He sent me the following comments about venturi design and as a result we built a couple of pages about his design and the PDF article that Zik wrote.

This is what Zik said:

I have been making venturis for a few years now and have a very nice, easy to construct, adjustable venturi design that is scalable to fit many applications. It is able to easily draw to 2 feet depth even with a low head pressure pump such as a Danner mag drive model 7 (700gph @0 foot’ head). For this I use a ½” inch version. For in our main pond I use a mag drive 1800 with a 1” inch diameter venturi. Running at 2 feet or more really makes a difference in the amount of O2 that can be dissolved into the water (due to pressure at increased depth and longer contact of the air with the water as it raises to the surface).

As far as efficiency is concerned, venturis are amongst the most efficient ways to dissolve O2 into water, when they are designed and applied right. You may wish to look at this page as it is very informative. I have been to the pages you referenced on your website before, as well as many others including some that do not exist at this time. (Wish that wouldn’t happen!!)

If you are interested, I can send to you a copy of an article I have written on venturi design. I have it in adobe PDF format for easy viewing and printing. It is not a perfect article, but I did revise it a few times so it is done pretty well. Since that time I have taken to making larger venturis of basically the same design. You wouldn’t believe the amount of aeration you can get from a mag 1800 / 1 inch” venturi combo. I was in the middle of building and testing that venturi when I was writing the article, so it does not appear in the article. If you wish, I could take one of them apart and take pictures for you.

I understand very well the head pressure trade off vs. amount of work done. Poor design can rob you of a lot of head pressure which boils down to wasting energy from the pump. I would be interested in what you think of the design I have, if you care to build it and test one out. It would be interesting to know if it works as much better for your application as I think it will, as long as the flow rate isn’t more than your filter can use…

Following on Zik and I had several detailed discussions about improvements to venturis and bio-filters. Anyway the page we created can be seen at our Basic Venturi construction for the DIY crowd page, and you can also download the PDF document he refers to which contains pictures showing a novel tooth design to make a venturi more efficient.

Thursday, October 27, 2005

How to prevent backwash from pond filters

I recently got a message from someone asking about using a venturi to prevent backwash occuring (dirty water being syphoned back from the filter into the pond) and thought other people might find it useful......

Hey Jim: printed out your pages on the Skippy and venturi and wound up with a question. I have been concerned with power failures in our area and need to prevent backwash from the filter. I have had this happen twice and don't want to risk it again. I have been fitting together various connectors to see which is more efficient and since a venturi would take up so little space in the plumbing I was wondering if it would be enough to kill the siphon effect. Of course the P connection would probably work as well. Before I start with the glue I wanted to check with you to see if my idea is something you tried. At the point where the tubing would take a 90 degree turn I would use a tee rather than an elbow, place a cap on the vertical opening, drill a hole for the air tubing and pack with expandable foam. I hope I have made this clear enough. I find I am not always clearto people who aren't used to me. Thanks, Sandy
Hi Sandy,

I'm glad you found my pages interesting.

I'm not dead clear on what you mean, but anyway here goes! Whenever I turn off the pump then the venturi makes quite a nice rude sound as the air is drawn in, so preventing any syphoning back from the filter into the pond. My bio-filter stays full of water. [During normal operation with the pump running, the venturi makes a constant quiet sucking-glooping sound as it takes air in and down into the bio-filter.]

Air will always move faster through an opening than water, so despite the air intake being quite small it is enough to prevent the syphoning backwash. This is quite handy because I know I can turn off the pump and it won't drain the bio-filter dry. Its also worth mentioning that before I fitted the venturi the syphoning would COMPLETELY empty the bio-filter! If this were to happen during the day, un-noticed due to a power failure, not only would it backwash into the pond, but also the bacteria on the media in the bio-filter might die - I think they can only stand a couple of hours without water, and then of course you're back to stage one with maturing your bio-filter all over again, which depending on the amount of dead bacteria could take a few weeks again!! Not good.

When you talk about replacing the 90 degree elbow with a T-piece, do you mean the bend is turning downwards? Because the venturi effect won't work going round a bend (I presume you do want the venturi to suck air in during normal operation, while air is drawn backwards to prevent backwash in the event of a power failure). The water flow must be straight through to create the venturi vacuum to suck the air in, as seen in the photos and the design diagram of my venturi.

Does this answer your question?

Your point about backwashing is a good one, and I will add something about it to my web pages.


Saturday, October 22, 2005

Milky or Cloudy Pond Water

Around this time of year (autumn in the UK) people often complain of the water in their pond turning cloudy or milky. You may get changes in water quality throughout the year, but it can usually happen when summer changes to autumn. First, its the time of year when the temperature is dropping down and the cycle in your pond is going to slow down and change.

I experienced some milky water for a couple of different reasons, but if you are experiencing this problem consider the following points:-
  1. You have given your filter setup a really good shake-up and cleanout. This can result in some fine particles going into the pond. This is cloudiness due to sediment and particles which have become waterborne.
  2. Having been disturbed by the filter cleanup, some of the good bacteria in the filter can die off and weaken its process on cleaning the pond water and keeping it clear. Usually this can be remedied by adding some more bacteria culture, although at this late time of year the bacterial processes will be slowing down and dieing off themselves anyway.
  3. Green water is associated with too much algae - an algal bloom, and Brown water is associated with too much sediment or particles. Milky water is usually due to algal bloom "die-offs", e.g. temperature too low for the algae to live so it dies. So with autumn upon us your pond has now cooled down sufficiently to cause a die off.
  4. Have you changed your food? Around this time of year when the temperature changes and goes below 10 degrees Celsius the fish cannot digest the usual summer food pellets. You should change to a wheatgerm based winter feed and slow feeding right down as the winter months draw in, until at some point the fish simply will not want to eat any more food. Continueing to use the summer food isputting too much nutrient into the water which may also exacerbate the milky water problem.
  5. Another reason for milky or cloudy pond water is if you have had excessive rain. Again, autumn brings changes, more rainfall being one of them. This may have caused a run-off ofdirt, clay or even chemicals into the pond from your garden and so could account for the change in water quality.
  6. You will also find at this time of year that fish change their feeding habits from eating the large summer food pellets as mentioned in point 4 above (so they leave it uneaten and it just rots and causes milky water). When they have slowed right down on their eating, as mentioned you should change to a winter food based on wheatgerm. But also now the fish tend to grub around at the bottom of the pond, looking for small, fine particles or grubs, little critters etc (I guess they know best what to eat to prepare for winter). Anyway - all that grubbing around, especially from the larger fish, does a lot of stirring up of the muck at the bottom, and if you have a venturi in the pond this will mix up the sediment in the pond, giving the water a cloudy appearance.
It will be best to concentrate on likely reasons for the milky or cloudy water being the feeding and the temperature change as the problem, rather than your filter, provided you have been continueing a normal routine for cleaning the filter. If you do something drastic to clean the filter too much then you will probably kill off the bacteria, and at this time of year they will have a hard time at recovering. So perhaps leave the filter alone (unless it looks incredibly dirty), because you want the filter to continue cleaning up any algae bloom die-off. Normally I try to leave a major dis-assemble and cleaning of the filter until the water has become quite cold and all bacterial processes will have died off completely.

Hopefully this will help explain a number of possible causes for the condition.

Monday, October 10, 2005

What matters? Clean or Clear?

Today I'm going to briefly discuss what is important about filtering your pond water.

Most people come to my web site to cure a "green water" problem in their pond. Actually green water is not necessarily a bad thing, but it can be an indication of more important problems. Green water is in fact simply nature trying to bring the ponds eco-system back in balance. But usually people want to get rid of the green water because it is unsightly and they cannot see their fish. After all what's the point of keeping fish if you can't see them?

So your concerns are more on an aesthetic level initially. You want clear water. Well that's fine. Yes of course you want to see your fish again, but you need to realise the more important reason for filtering your pond water.

Clean water is more important than clear water!

A biological filter is quite simply the heart of a koi pond. It is not essential in small fish ponds, but the more fish you stock, the larger they get and the more they eat, so the need for a bio-filter becomes greater. The pond gets to a point where it needs a "sewage farm". It's purpose is to convert the waste matter produced by the koi from harmful ammonia into less toxic waste.

It is less important to remove solids particles from water than it is to process nitrogen, so if there is to be a compromise between mechanical and biological, err on the side of biological.

In other words, it is much better to allow sediment particles below a certain size to escape back into the pond, while converting a great deal of ammonia to nitrate, than it is to catch every little thing down to a micron or less which in the process would slow the water down to the point where the bacteria have a hard time living (because they're not getting enough oxygen).

The bacteria that convert ammonia to nitrate for us are among a class of bacteria that you may have heard of before. They are the so-called, “nitrogen fixing” bacteria. This means that they take nitrogen that is unavailable to plants in its ammoniacal form, and make it available to plants in an oxidized form.

These are the same bacteria that live among the roots of leguminous plants. Without these beneficial bacteria, life as we know it would cease. So be kind to your bacteria. What they need to survive is a large surface area, chemically inert medium and a ready supply of fresh water. They depend upon dissolved oxygen in the water to live and to do their job. As soon as the water flow is stopped, the oxygen in the filter becomes finite, and eventually gets used up. The ultimate result is that the bacteria die, and you have to start over.

By using and maintaining a bio-filter to keep it in good running order you are ensuring that the water in your pond remains clean.

And the by-product of having clean water will in time result in clear water, and happy, healthy fish.

Tuesday, October 04, 2005

Using a Venturi to oxygenate a Bio-Filter

Shortly after I built my bio-filter, I had been reading on my favourite fish pond forums the benfits that a venturi can give to your fish in a pond. Depending on the size of pump you use, it will provide a strong current for your fish to swim against. This gives them some exercise. My fish love to swim directly into it, or play in the bubbles.
The main benefit though is that oxygen is being added into the pond water. This is quite important for a number of reasons; it helps your fish breath (quite important!), especially in summer and at night-time when plant bio-activity reverses and absorbs oxygen, and the oxygen helps good-bacteria do their work of breaking down ammonia.

Well I figured that if a venturi is good for your pond, why not use one feeding directly into the bio-filter to oxygenate the water where you want bacteria to do most of their work.

Here is a diagram showing the insides of the venturi.

You will find lots of information and photos on how I designed and got my venturi working on my web site.

Sunday, October 02, 2005

Fish Pond Bio-Filter Essentials

Did you know that us humans convert the ammonia in our bodies into urine, whereas fish simply excrete it continuously from their gills into the surrounding water?

Normally in a river or the sea it is diluted by thousands of gallons of water to render it harmless. But nobody told Mother Nature about koi-keepers and their ponds, where ammonia can build up to a dangerous level due to the large number of fish in a small volume of water.

A koi pond has to deal with two types of pollution;
  • solids waste and
  • dissolved waste from solids.
Therefore it is essential to remove the solid wastes from the water before they have a chance to dissolve.

If we can do this we gain; better water quality, fewer dissolved pollutants and ultimately less fish health problems :o)

Once the solid wastes have been collected, it is important that they are flushed out of the system regularly, before they get the chance to decompose.

In summertime this could be as often as twice a day! This means that any settlement chamber incorporated into the filter design will need to have a drain to allow easy flushing to waste.

It doesn't matter whether the solids decompose in the pond or the filter - the result is the same - polluted water!

To maintain good water quality it is essential that solids are removed from the pond and filter before they have time to pollute the water

Any trapped solids must be removed from the system on a regular basis, otherwise they will simply decompose and pollute the pond. They will also encourage high levels of opportunistic bacteria.

For good filtration and water quality very little solid waste should be allowed to enter the 'biological' section of the filter. To restate the point made previously; the more effective the settlement area of the filter at removing solid waste, the lighter the load on the following biological section - provided, of course, that trapped waste is removed before it decomposes.

This entails:-

Regular maintenance to keep the biological area clean and free of mulm,

Reducing the level of dissolved organic compounds by effective settlement/entrapment, together with regular cleaning of the settlement area.

If we can remove solids from the system before they decompose and at the same time keep the biological section of the filter fairly clean we will;

  • Encourage a vigorous growth of nitrifying bacteria
  • Reduce the load on the biological section
Do you see a repeated theme here? Cleanliness is the order of the day!

Biological Filtration and the Nitrification Cycle

So how does a bio-filter work?

A biological filter is quite simply the heart of a koi pond. It is not essential in small fish ponds, but the more fish you stock, the larger they get and the more they eat, so the need for a bio-filter becomes greater. The pond gets to a point where it needs a "sewage farm". It's purpose is to convert the waste matter produced by the koi from harmful ammonia into less toxic waste.

There are 2 types of bacterial species that colonise the biological filter media.
Nitrosomonas sp.
bacteria which oxidize ammonia to nitrite,
Nitrobacter bacteria convert nitrite to nitrate.


Ammonia (NH3) is produced by fish (and particularly koi because they are fat greedy chaps!), as part of their normal metabolic function and is excreted from the gills. The amount of ammonia produced is directly related to the amount of food they eat. Approximately 3-4% of normal 30-40% protein level koi food will be excreted as ammonia, i.e. for every 100grams of food 3-4grams (3000-4000mg) of ammonia is produced.

Koi exposed to unacceptable levels of ammonia risk damage to gills, eyes, fins and skin which can result in them being susceptible to secondary bacterial infection.


Ammonia is oxidized by the Nitrosomonas sp. bacteria in the filter to produce nitrite (NO2).

Whilst it is not considered as dangerous as ammonia it can still do serious damage to your fish. High levels of nitrite are likely to stress your koi leaving them susceptible to secondary infection. As with ammonia, target levels should be that nitrite is undetectable.

Before the fish pond filter can efficiently remove ammonia and nitrite from the fish pond water, it must first become fully colonized with nitrifying bacteria. This can take some time and is a process known as fish pond filter "maturation". Each time a fish is put in the fish pond it will add to the total amount of ammonia being produced. The ammonia level in the fish pond will therefore increase slightly. Because there is more ammonia for the bacteria to utilize, they start to multiply until there are enough to use all of the ammonia being produced inside the fish pond. The ammonia level in your fish pond will then fall back to zero.


As the ammonia level falls, the amount of nitrite produced by the bacteria in the fish pond filter will start to increase. Therefore, the level of nitrite in the fish pond will rise. The increasing nitrite level means that the bacteria that break it down can start to multiply in the fish pond filter until, as with the ammonia, there are enough to use up all the nitrite that is being produced. The nitrite level within the fish pond can then fall to zero. As this occurs, the nitrate level increases.

Conversion of nitrite to nitrate (NO3) is the final stage of the nitrification process. There is debate as to the possible problems that elevated levels of nitrate may cause. Indeed some koi keepers have high Nitrate and it causes no problem at all. High nitrate may also attribute to green water (phytoplankton) and blanketweed growth however the two do not always go hand in hand. The green water problem can get worst when you clean the biofilter and make water change outs, due to the reduction in bacteria.

The bacteria also produces a certain phytoplankton-killing enzyme. As algae starts to grow in the bio-filter, or on the walls of the pond, the bacteria loves to feed on this algae, and as it does so it releases the enzyme into the water.

Green water is a pain for many reasons. Ultra Violet Clarifier lights will kill single cell phytoplankton algae that cause green water, and when dead they clump together and can be removed by the filter. However there is sometimes a concern expressed that passing water through the UVC also kills beneficial bacteria. Note that a UVC does not get rid of blanketweed.

The Skippy site teaches us that we should try to achieve "balance" in the pond - don't fight mother nature.

By use of the bio-filter and other larger plant forms you starve the water of Nitrate, so that the algae has no food, and is therefore unable to grow, while at the same time the bacteria create the enzyme which kills the phytoplankton. Its a double-edged sword in this battle!