Sounds like some sort of freshwater crustacean, possibly Cypris. These are harmless and usually show up in water with lots of nutrients. Unfortunately they are covered by a very hard shell and few fish will eat them. Do you vacuum your gravel regularly to cut down on the build-up of fish waste and uneaten food? You might give that a try if you don't already do it.
If you see these small black dots attached to your fish then you're dealing with something completely different...the fish louse, Argulus. These little monsters are also crustaceans but they are parasitic and can kill fish outright or leave them susceptible to bacterial infections. If this is the case in your aquarium then I recommend a product called Jungle Parasite Clear which will get rid of the Fish Lice pretty effectively. If you use this product, be sure you take any desirable invertebrates like shrimp or snails out of the aquarium before treatment.
