Clamped fins is usually the first sign of stress. The stress can be due to many factors, a few being disease and water quality. What are your water parameters? Is the loach clamping all of its fins, or just its tail?
I do a water change every week too, yet somehow my nitrates were up over 50 the other day. I only found this out after losing 6 angelfish and some neons. My at home tests were faulty
I'd get some tests and check ammonia, nitrite and nitrate asap.