OK. I would co for an internal filter like the Aqueon Silent Flow (pictured below, the box is the 10g size, they come in 3 or 4 siz\es so you can get a 40g size which would suit things, that is what I have). I have one of these in my 40g which is the same length/width, just less vertically. I have 40-odd wild cories (a few fry too) including my pandas and this filter placed in the right rear corner with the flow directed down the back of the tank has worked very well. The tetras and pencilfish have no issues, and the copies seem quite satisfied. I even used this in the 4-foot 70g tank for a couple years after the canister gave out.
