I too have some black kuli loaches and I definitely love their personality, but I do wish they were a bit more visible, so I see where you’re coming from. As for wether you should buy the fish, personally I would say yes, but not from this source. Aquhuna has striped kuli loaches for $3 each + $12 flat rate shipping I believe. I have a feeling you will probably save money because you will probably loose less fish and save money on meds too. As for quarantining the fish, I would probably just fill your 15 gallon up with 10 gallons to make the dosing easier and then you will still be able to observe the fish and it will save you from setting up a at tub.