I just increased my school size of rummynose tetras to 16. They are currently in a 65 gallon high, which has a footprint of 18x36. I'm wondering if this is too small for them. They seem to be swimming back and forth a lot and I'm afraid that the length might be too short and that they might need more swimming space for their school size. Is a school of 16 in a 36x18 tank okay or is it too small?