I have two yoyo loaches i got recently for my 26 gallon. (there were only 2)
anyway, the bigger one 6cm is quite happy and healthy still hides but each day i see him a bit more. The other 4cm one is gay....well hes quite annoying cause hes either wedged between rocks or lying on his side breathing heavily in the corner. He hardly ever, actually never has swam without being chased with the net i used the afternoon to move him to a 5 gallon substrate, though all he does is ly down and breath heavily.

My water is fine, and the other fish 5 tiger barbs and 3 swordtails. + the other loach.

What could be the problem?

Sounds to me like he may be the victim of bullying. Do you notice the other fish chasing him or pieces missing from his fins? Even fish that are smaller than him can bully him. The only thing I know to do is move him, which you have already done. I would just keep him in a separate tank with the light off for a while until his condition improves, if it does. He could just be really really stressed. He may die anyway. I'm sorry if this wasn't very helpful, I don't know why else one fish would be acting sad and the rest acting fine. :(

Tbh with you it could be bullying but more likely a more serious problem. When did you get the water checked and how was it with test strips? because if it was they are useless you need to use proper drops test kits and if its at your local shop go somewhere else if their using strips. pets question and answers,www.5d2d.com

Because this to me sounds just like a water quality issue probably with the amonia as high levels of amonia burn the fishes gills which in return will stress them out and make their breathing heavy.

Theirs also a good chance he could have some kind of internal bacterial infection or even swim bladder if he cant swim so i recoment adding a general tonic that can treat a good few types of illnesses.

Also as a precursion i would add freshwater aquarium salt you can get it from nearly all pet shops its not the marine one, follow the instructions and it will tell you corect dosage. Marine salt is great for tackling most problems in fish thats why alot of freshwater aquarists keep small salt lvls in their aquariums.

Good look with it