10 kuhlis in a 15 gallon already stocked tank? That seems like too many fish in a small tank. I wouldn't do 15 kuhlis in a 15 gallon tank, either...that's a heavy bioload even if they're thin, low bioloadish fisn.
I also disagree that you need 10 to enjoy them...I have a trio of black kuhlis in a 20 long. I'd like to add 3 more, but I'm waiting for some elderly variatus platys and harlequin rasboras to pass on to fee up bioload (other tank mates are my black paradise fish and a clown pleco).
Anyway...my 3 are active and out and about all day. Granted, the black kuhlis seem much more diurnal than standard kuhlis...but in a well planted tank, kuhlis tend to be rather active even in smaller numbers.
Just my $.02 worth...YMMV.
Eric