What to feed clown loaches

I bought 3 clown loaches about 9 months ago when I had a snail problem.
They did a number on them and now they've been eating the reguluar pellet
and flake food I feed my other fish (mainly tetras, platy's etc). I also
notice they like to chew to the algae wafers I throw in occasionally for my
plecos.

However, last night when I was doing my bi-weekly cleaning, I was looking
down from above in the tank while vacuuming with my water python and I
noticed one of the clown loaches was literally paper thin side to side.
There's no meat on his bones at all. The other 2 look fine.

Is it that they're not getting the proper nutrition after all from the other
stuff and it's just showing on one more than the others ? Or maybe this one
has a parasite or something.

Any ideas ?