Clown Loach

[holidayinn]

Hi, not really sure if this is a real emergency, but it is something that has been bugging me. A couple of weeks ago I purchased a number of clown loaches about 6 altogether, all between 2-3" in size. They all seem perfectly healthy, active, eating etc. However one of them seems to have black eyes. The whole of both eyes seem black. All other clowns have standard eyes, nothing unusual. I'll try to get pics at some stage, but they are forever hiding. Does anyone know what this could be?

Btw, Ammonia 0, nitrite 0, nitrates registering...twice weekly waterchange about 25%, I have separated this clown from the main tank and placed him and another clown in a 2 foot tank, so I can track progress. Other occupants include some rasboras, tetras and cories.

Thanks for any advice