help!

Honestly, a 50 gal is generally considered too small for clown loaches.

I agree... depending on the size of the group, 100 gallons may even be too small for adult CLs.

You could very well have ich in your main tank, it could just be in a stage of its life cycle that isn't visible to you. The safest thing to do would have been to treat the main tank. I'd definitely keep an eye on those fish.

What's a UV light filter? A UV sterilizer? And why assume it worked for all fish when clearly it didn't work for the loaches?
 
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