By Scott W. Michael
Q. I am a new reader of Aquarium Fish International magazine, and I am quite satisfied with it. As any other reader, however, I have several questions regarding the up keep of my marine aquarium. I have a 65-gallon tank with a canister filter, and I am about to install a pair of powerheads on the undergravel filter. My intention is to use one of powerheads in the normal flow, and, because it is very powerful and feature reverse flow capability, use the second one in reverse.
This is because such as setup supposedly strengthens existing bacteria populations in the gravel bed and also stimulates a bacteria colony under the gravel plate. I talked to my dealer about it. Despite his experience, he said he has never done it, but maybe it would be worth a try. I have presently five small and one medium fish, and I intend to add two or three small ones. What do you think?
A. This is a good question. Reverse-flow filtration was becoming quite popular before the wet-dry filter came on the scene. The advantages of setting up a filter in the reverse flow manner is that it enhances biological filtration.
With a conventional undergravel filter, only the upper surface of the substrate develops a rich nitrifying bacteria colony. But, by pushing oxygenated water under the filter bed, the lower surface of the substrate will develop a healthy growth as well. The growth of bacteria on the upper surfaces of the gravel bed is a result of its exposure to the oxygenated water that is moving about the interior of the tank. Another advantage to encouraging nitrifying bacteria growth on the substrate surface nearest to the undergravel plate is that these bacteria will not have to compete for growing surfaces with microalgae because light does not reach the lower regions of the gravel bed.
One thing you will need to do if you are going to set-up a reverse-flow undergravel filter is to pre-filter the water of larger particulates before you pump it under the undergravel plate. To do this you can attach a sponge filter to the intake opening of the powerhead, or you can use a canister filter to pump filtered water down the undergravel uplift tube. If you don't filter the water before pushing it under the filter plate, the plate is more likely to get clogged by larger particulates. Water will then be channeled through the areas where the filter bed is not clogged.
One way to ensure that the water is flowing evenly through the entire filter bed is to clean the filter bed regularly with a gravel vacuum. With an undergravel filter is configured in a reverse-flow setup, the filter bed should not clog as readily as it does in the conventional configuration. The pre-filtered water being pushed under the undergravel plate will minimize this problem.
The other thing to consider before you set up your filter is the configuration of the undergravel plates themselves. If the tank has one large plate, then you will want to set both powerheads so that they are pushing water under the filter plate. If there is only one plate, and water is being pushed down (reverse flow) one lift tube and being pulled up a second lift tube in the normal way, the water that is pushed under the plate by one powerhead is going to be pumped out from under the plate by the other one. The water will neither be pushed up or pulled down through the substrate, and the filter will not work effectively!