I am having a possible similar situation to this.
I have a new tank, I just cycled in. It's about 2 months going on +plus 1 week.
I also just introduced a clownfish into my tank. It's been in it's new home now for a little more than a week.
Anyway I thought I noticed a few slight discolorations in it's red body coat? I am pretty sure I am seeing this right, but I am not sure if that is just normal for clownfish to not have a perfect red coat, or could be something else.
I am using all RO/DI water. Besides the clownfish there are a few invertebrates that have been in the tank just a bit longer than this clownfish. Say about 2 weeks more than the clownfish.
I have a 20 gallon L with about 18 lbs of cured live rock. The aquarium temperature is 82 degrees and has been roughly this value for over a week now if not more.
I also have one canister filter Hob filter (meant for a 30 gallon tank), a powerhead, 500gph and a bubble wall emitting air bubbles.
Anyway not sure then what might have caused a possible parasite infection. Again I'm not even sure if it really has one at this point.
However I've been noticing strange behavior from my clownfish. So far it has been swimming when I put the aquarium light on, and seems now to get up around 10pm and swim around the tank over and over again until 8am.
Anyway, I have noticed it as it swims toward the left side of the aquarium, it like hangs out there for a bit, moving around and looking through the glass.
Two possible reasons for this popped in my head?
1. It thinks it sees another clownfish (itself through the glass) and is trying to play with it a bit?
2. It is actually trying to rub against the glass for some reason?
Yet if this was the case I would expect this behavior wherever it swims along the glass, but it is just doing this "catch me if you can" strokes on the left side of the aquarium glass?