Sponge filters are cheap and good for fry tanks because the fry cant get sucked up. I had a HOB filter in 20 long Fire Mouth fry tank with a net over the intake and that worked also.
Petsmart now has a new filter that sticks to the glass and has a sponge inside with a spray bar on top. I am using this on my 40 breeder fro my Honduran Red Point fry and it seems to be working well. I forget the make of it.