It doesn't matter how good the filter works, if the pond and water delivery system is not correct for the pond size and shape you will not remove all the toxins from the water.

First, I want to talk about turn over rates. Turn over rate is how long it takes for the entire volume of the pond to go through the filter. Although you will read that the ideal turn over rate is every 2-4 hours, this is just part of the story. For example, if your pond is 10'x15' and approximately 3' deep at the deepest point, it holds about 2000 gallons. There is a waterfall at one end, which is the overflow from the filter, and the other end has a skimmer, which is the inlet to the pump. This is the only movement of water in the pond. The water goes from the skimmer to the biological filter and then to the waterfall. With our stated turn over rate of 2-4 hours, we would need to deliver 500-1000 gallons per hour to our biological filter. Let's say the top of the filter is 2' above the pond water level so the water is pumped to a height of 2'. You might look for a pump that can pump 500 gallons per hour at 2' head pressure. In this setup the pump would probably only deliver about 300 gal/hr in actual practice. This is due to the friction of the water moving through the pipes to the filter. Because of this friction, you actually need a pump that can supply the desired amount of water at a higher head pressure. If you want to know what the required higher pressures are, there are several articles that will explain how to calculate the friction losses.

Getting back to our example, this is a poorly designed pond for fish, although it is just fine for plants. The reason is, even if you have the proper turn over rates, you are turning over only a small portion of the pond. In this example, only the small corridor of water from the waterfall to the skimmer is being turned over and very shallowly at that. The rest of this pond is basically dead. Toxic levels will be very high. This will distress the fish in this pond.

What can be done in this example? One thing would be to add air stones to the dead areas to mix up the water. Add another pump in the pond to move the water around. The biggest thing would have been in the initial design of the pond. Everyone tries to design a pond with the skimmer straight across from the waterfall. This may or may not be the best design. It really depends upon the total design. Turn over rate means that all the water in the pond must be turned over in the stated period of time and not just the same small amount of water.

If you plan on having fish in your pond, you must take some water off the bottom of the pond. This should ideally happen all the time as with a bottom drain or under gravel filter. If this isn't possible, you will have to remove some of the water from the bottom at least once a day. This could be as simple as an air stone on the bottom forcing the bottom water to be circulated. Or you could put a pump on the bottom for water circulation or to pump a filter. Or you could remove the water from the bottom when you do your water changes.

Let's talk about water changes. Yes, there is no such thing as a pond that is designed so well that water doesn't have to be changed. In the water in a pond there are all kinds of chemicals and minerals that are used up by the fish and the filter. If these are not replaced by water changes, both the fish and filter will suffer. This does not mean just replacing the water that evaporates. When water from the pond evaporates, it leaves behind everything that was in the water. This includes toxins from the fish and filter. Some of these need to be removed by water changes before the concentration gets too high and not just diluted by topping off the pond for evaporated water.

The last item I want to discuss is foam fractionators. Foam fractionators remove the excess protein compounds from the pond. If you have ever seen waves on the ocean or large lakes come ashore, you have seen this brown foaming mess being deposited on the shore. Excess protein compounds cause this. We don't normally have waves strong enough on our ponds to create this effect. So we handle it with a foam fractionator.

How do you know if you need one? If any bubbles from your waterfall go more than one foot from the waterfall without breaking, you have excess protein matter building up in your pond. This is just one more thing that distresses the fish.

These articles I have written are a very brief overview of filtration, but I hope that they have helped you solve some of your pond problems. If you have any other questions, feel free to email me.