Well I guess I should have read a bit more about bettas before making
the suggestion. I did a little research just now and from what I can
see they can be kept together but not always with perfect results. Sort
of a ymmv situation. Probably not a good idea to just throw one in and
hope for the best. But I do think a second fish would solve the problem
better than adding nitrates directly. Maybe a couple small corys or
kuhlis would do nicely. Or if that's too much load perhaps one of the
pl*co strains that don't grow much...I've always liked clown pl*cos and
they wouldn't outgrow a tank this size I don't think. Also a pl*co
would immediately solve the algae problem which would increase the
available nitrates for the rest of the tank.

-Daniel