Charles wrote:

Sounds like gammarus. Try an imaged search on Google.

They can be hard on plants. Some fish eat them, others won't, or
won't eat enough to control them.

Wow, thanks Charles! I've seen those buggers from time to time, and always
thought my Amano Shrimp were occasionally breeding.

Between loaches, ancistrus, tetras, angels and otos, I guess the gammarus
population here is kept in check. But next time I find them in the filter,
I won't put them back in the tank!