Q. I am a new fish hobbyist and find your column both interesting and helpful. I have a 29-gallon tank with an undergravel filter and a hang-on-the-back power filter. I have approximately 25 tetras in the tank. My problem is that the tank water is often somewhat cloudy. Temperature and pH are consistent, and the water test kits I use do not indicate anything unusual. Every week I vacuum the gravel in the tank while removing and replacing 6 gallons of water, and I change the filter medium. Despite this, the water is never as clear as in the aquariums at my dealer. Am I cleaning the tank too much? What am I doing wrong?

A. Your techniques and basic setup seem very good. Being limited to the information you supplied in your letter, your problem is somewhat of a mystery to me. There are, however, a few thoughts that come to mind that may have a bearing on your case.

What color is the cloudy water? If it is grayish, it's probably a bacterial bloom. If it is greenish, it's likely to be algae suspended in the water. Bacterial blooms are usually caused by excessive nutrients in the water, and large amounts of algae are often the result of too much light in the tank.

Unless the water is green in color, the problem is most likely a bacterial bloom. What are you feeding the tetras? Excess dry food is the prime cause of cloudy water. If you use dry food, several small feedings are better than one large feeding each day. You might consider adding a few bottom-feeding fish, such as catfish, to your tank to help clean up uneaten food. Bottom feeders are not scavengers, however, and should receive adequate nutrition in addition to the scraps they may locate in the gravel.

If the tank is newly set up, the bacterial bloom may be the result of the biological filter (the undergravel filter) not having a sufficient population of bacteria. It takes as little as two months and as long as three or four months for the bacterial population in the gravel to reach a sufficient level at which the water remains crystal clear.

Finally, how deep is the gravel bed? Although as little as 1 inch of gravel may be sufficient, it may require as much as 3 inches of gravel to provide enough surface area for the bacteria to colonize is sufficient numbers.