Yes I know I am comming off as a big worry wort... but I am.. so here goes.
today 2 of my 6 Cory Cats have been just lying in one spot for a very long type. They are alive, I can see them moving, but they are just lying there.
The other 4 are active as usual. All 6 are usually active.
I read on another site that the two not moving might be females, because they tend to lie in one spot on not move from time to time, but I am no expert and have no idea if this is true.
Today I added a small Canister filter (only rated for a 20 gallon, on my 29g) to back up my HOB filter, because I didn't think there was enough filtration or biological media.