Are they half moon shaped if so here is some info. I found this under invertebrate pests in my book.
Amphipods
Amphipods are small crustaceans generally found feeding in the detritus at the bottom of an aquarium within the filters, or even on the front glass. They are easily identified as laterlly compressed grey shrimp like creatures, half moon in shape.
Amphipods rarely do any direct damage;however, some species may carry didease which they can pass on to other livestock.They are quite hardy and capable of multiplying into plague proportions when there is enough food, usually privided by an overenthusiastic hobbyist.
THE SOLUTION: An immediate cure is to remove the creatures by syphoning them out of the tank. They will be found under rocks during the daytime and quick action will be necessary to allow for their surprising turn of speed. In the long term, a drastic reduction in the food supply will see a steady decline in numbers until they are barely noticeably. Many fish would enjow copepods as a food if they had access to their hiding places. Regular disturbance of the rockwork may reduce numbers as the fish learn to associate such activity with a potential meal.
Hope this helps.
Adam
