I have 6 long-fin zebra danios in my 15 gallon tank, and I keep it at 73. What single fish (so not a school of 5-6 fish) would be a good tankmate for them? I've thought about a dwarf gourami, but any other ideas? I believe they would nip at a betta's fins, so that is probably out, although I'd love to add a betta to it. Not looking for a bottom-feeder or any type of loach/catfish/etc. Inverts would be a last resort.