Q. I have a 33-gallon flat back hex fish aquarium that contains five goldfish: a black moor, a red cap, a ryukin, a shubunkin and a lionhead. They are all about 4½ inches long. The aquarium filter is a wet/dry filter along with an undergravel filter with a powerhead in each corner. There are also two airstones in each corner and two devices that produce bubbles.

Here are my questions. My lionhead goldfish turns upside down on its back after I feed it and remains in that position for several hours. What is the problem?

After feeding time all the other fish are on the surface gulping or gasping. Sometimes this happens later in the day as well.

Water tests show that I have no ammonia, although there is nitrite (though not enough in my opinion to harm them). I feed them dry pellets once a day — about a tablespoon. Once a week I feed the fish shrimp and blood worms.

A. The cause of your lionhead's problem and the solution is the dry pellets. Soak them for a few minutes before feeding. Also break the feedings into three portions per day — without increasing the total amount of fish food.

The fish are gasping for oxygen. You simply have far too many fish in too small an aquarium. Fish oxygen consumption increases significantly during feeding, as does ammonia production. It seems likely that the nitrite levels are too high as well, despite your opinion that they are not. Nitrite enters the blood and forms a compound that hinders oxygen uptake. Heavy feeding, an overloaded tank and nitrite combine to create hypoxic conditions.

If the situation continues the fish will succumb to disease brought on by stress. First, I would immediately add 4 ounces of salt (about 8 tablespoons). This will alleviate the nitrite poisoning.

Second, remove all but two of the fish. Yes, that's correct — just two. (Frankly, I would only keep one in that tank, but you would never follow that advice. Hex tanks have less surface area for the same volume of water than regular aquariums.)

Next, I would reduce the amount of filtration in the tank. You do not need a wet/dry filter and an undergravel filter and the bubblers. I would remove the undergravel filter, which duplicates the wet/dry filter. If you have tank heaters, shut them off. Your goldfish do not need heated water.

If you follow my advice things will return to normal and the fish will stay healthy. If you choose to ignore my suggestions, please write back in six months and tell us what happened.