Normal fertalizers are fine, just think of water in the wild (it has all the same minerals etc in it).
Clown Loaches don't have scales that act as "barrier" as such, so if something CAN be toxic to fish by overdosing like adding medication or adding certain fertilizers that have a set dosage you should half the the dose to allow for the clown loaches sensitivity.
Most potentially harmfull additives these days have warnings on the back about following the set dosage.
Also, if in doubt avoid using random brands and stick to the more common well known brands such as: SeaChem, API, Sera, aqua one etc etc
Its a bit like removing an oven mit from one hand, and then removing a hot tray from an oven. One hand handles it fine, the other not so well :lol: