Derasa problem

[campaign000]

Here is my friend's tank using 4 T5s, and he got the big derasa 3 months ago.
In the beginning it's thriving but then mantle less extended and tears appeared (looks like rot). The water wasn't good enough back then (No3 50, po4 0.5).
He improved water quality and did FWD twice 2 weeks ago, but there's no sign of improvement or deterioration.
Any idea?

 

[Troy V]

Nice tank!

It looks like a fish is nipping at the mantle. Also the excurrent siphon is retracted like something was biting it.

 

[campaign000]

Nice tank!

It looks like a fish is nipping at the mantle. Also the excurrent siphon is retracted like something was biting it.

Thanks for the input!
Find no nipping so far and will keep closer eyes on it. It's weird all other clams are fine, worried if it's bacteria thing.

 

[chuckfu5]

Wow! How big is that tank and how much alkalinity and calcium is he having to supplement?

That is a great looking tank!

 

[TheSchwenkster]

Agree on the tank, it is quite amazing!

Wish I had better input on the problem. If it hasn't gotten worse, I'd say stay on current course and see where it taked you.

 

[Troy V]

Thanks for the input!
Find no nipping so far and will keep closer eyes on it. It's weird all other clams are fine, worried if it's bacteria thing.

The spots you think are "tears" are actually bite marks. From my experience that is exactly what they look like. Also I've found the best time to catch them nipping is at dawn or dusk usually when there is just enough light to see in the tank. I bet that sailfin Tang is the culprit, and a stressed clam will add to this behavior.

 

[DeniseAndy]

Definitely nipping. What fish are with it besides the powder blue I see? I did not see the sailfin in the pics?

 

[Zos4me]

Amazing like an display for clams I love it