I've never had any luck keeping a "school" of them. I put 6 in my 90 and 5 in my 56. The 90 is down to 3 I think, and the 56 is down to 2. Been about 9 months I guess, since I last stocked them.
I'm always having to put "new" ones in to keep them schooling. Seems when they get down to 2 or...