1 Loach not growing

[Loach]

I bought 2 clown loaches that were the same size about 6 months ago. Now 1 is twice the size of the other and the smaller one looks skinny but eats his dinner as much as the others. What can I do?

[Diana]

Sounds like a common problem. There is a lot of info at this site.
You can search through the treads here, using the following words:
skinny disease
chronic wasting

Clown Loaches are often wild caught, and can pick up many different parasites and diseases. Best treatment is to quarantine and treat right away, using an alternating treatment of wormers and antibiotics.
By doing this in a quarantine tank before adding them to your main tank you avoid spreading possible parasites and diseases into the main tank.

If these Clowns are already in the main tank then you will need to treat the main tank, because whatever is wrong with the one Clown has had a chance to spread. The other fish may or may not have enough of an infection to show symptoms, but if you just removed the one and treated then put him back he could get sick again. Some fish are more resistant to parasites and diseases, and the other fish may not be showing symptoms.