I wondered about that but they were so little I thought it couldn't be.
pH was about 7 and it is a sponge filter so nowhere for them to go, and the substrate was fine gravel on one side, bare tank on the other.
I'll keep checking the ammonia and making sure I don't feed them too much.
My husband thought that microworms might cause it, because of the rotting porridge, but I've only been scraping them off the sides, not taking them from the base.