Q. I have two years experience in fishkeeping. For the last four months I have had two Monodactylus argenteus. They are housed by themselves in a 10-gallon aquarium. The tank is equipped with an undergravel filter and a power filter. The monos are in freshwater. My aquarium atlas has little information on these fish, but it does indicate that these fish can live in brackish water. I would like any information you can provide, and any books you can recommend, about brackish water aquariums, plants and fish. Should I change my water to brackish, and if so, should I do the change slowly? I am truly taken by these beautiful and timid fish. I would like to provide the best environment for my monos, and, in the future, try to breed them.

A. The most important thing I can tell you is that you will need much more space for your fish. Monos mature when about 8 inches in length and grow to approximately 10 inches. The natural range of the mono is throughout the Australo-Indo Pacific in estuary habitats, although they do venture into completely freshwater environments.

I have maintained individuals of these fish for years in aquariums with crushed coral substrates and freshwater that contained a tablespoon of rock salt per gallon of water. However, they grow more rapidly in seawater. Use a good-quality synthetic sea salt. I fed mine a diet consisting primarily of frozen krill and algae, but they ate anything — from prepared dry foods to live crickets or cut beef heart.

On the few occasions that monos have spawned for me, they deposited their eggs on solitary rocks on a sandy bottom. There did not appear to be any parental care. In fact, the parents ate some of the eggs and they preyed on the fry as soon as they were free-swimming. The fry were initially raised on rotifer and algae cultures until they were large enough to eat live brine shrimp. They begin to look like monos when about ½ inch in length, or about five weeks of age. When approximately 1 inch long, the ventral fins begin to disappear.