Sounds like the tank is going well
. There are sometimes multiple nitrite spikes so keep testing nitrite just in case. Is there any nitrate yet? That tends to mean you're really close.
I don't remember if you mentioned it but can you get kuhli loaches locally? Those are pretty common in U.S. pet stores, at least. If not, see if they have any other loaches...there are a few that stay small
. Another option would be to consider shrimp...a bunch of cherry shrimp would look nice and they're fun to watch. A single bristlenose pleco might be OK in a 15-gallon although they're kind of messy...might not make for a low-maintenance tank, ha.
Yeah, that's enough room for most HOB filters.