Q. I have a 10-gallon aquarium with a corner filter. The tank has been running for a number of years. The tank contains two dwarf gouramis, one coolie loach, one plecostomus, two bronze Corydoras catfish, two quarrelsome silver-tip tetras, two serpae tetras and a dwarf aquatic frog, along with 11 guppies (two of the guppies are males). The pH is around 7.6. How can I tell the sex between dwarf gouramis? I bought my gouramis for breeding. According to my fish book, I have a male and a female, but they both look like they are full of eggs. What do I do? Also, my guppies will no longer breed. How can I get them to start breeding again?

A. My first thought is that there are far too many fish in this tank for the size and type of filter. Corner filters are not the most efficient with a high biological load, and poor water quality could be the main reason for your fish not spawning. I hope you are using good aquarium maintenance practices, including frequent partial water changes, to offset the limited filtration that your corner filter is providing.

I think your lack of success in breeding your fish may also be due to the amount of activity in the tank that is preventing courtship rituals. If this were my aquarium, and I really wanted to breed any of the fish, I would give the 10-gallon tank to the gouramis, another 10-gallon tank to the guppies, get rid of the two quarrelsome silver-tip tetras (and maybe the plecostomus) and add six to eight other peaceful tetras to the community display of guppies.

The usual way to tell the sex of mature dwarf gouramis is by the intensity of the body color. Both sexes display vertical bars on the flanks, but there is a startling difference in the intensity of the colors in the male. However, there are many new selectively bred color varieties of the dwarf gourami on the market, and this difference between males and females may not hold true for all strains. What does hold true, regardless of strain, is the difference in the shape of the dorsal and anal fins. The fins of the male are sharply pointed at the rear of the fin, whereas the female's fins are rounded.

I have no way of determining if you have both sexes without actually seeing them. Although it is true that some males do eat enough to round out like an egg-laden female, this is quite rare. You can check for this by isolating both fish for three to four days without feeding them. If a fish is a male, he will slim down, whereas a female will not show much difference in this short period of time. You do not mention any other problems, so I will discount the possibility of an internal bacterial infection that could exhibit the same signs.

If you can get your guppies out of the hustle and bustle of the crowd, they will probably start breeding again, assuming that they are not too old. Good clean water never hurts either.