Do you know what happened to the tank?
You could do a trio of Endlers, even a trio of scarlet badis would work in that size tank. School of micro rasboras. Shrimp.
Long story short, nuked it with a bowlful of hydrogen peroxide.
I was actually looking at class N black bar Endlers from AdrianHD, but wasn't convinced about 5.5 vs recommendation of 10 gal. I very well could move the adults to the 40 if (when) they breed, let the babies grow out, move those, then put adults back. Don't know if the filtration could keep up with a full batch of fry, though. I suppose so, given that it kept up with a sorely overstocked tank until I killed all the BB.
Shrimp are in the 40, I could get another batch of blues and use the 5.5 as a grow out. Actually.. That sounds a marvelous idea and would allow me to get kuhli loaches for the 40. Theresa! You're a genius ?