Heniochus dorsal fin getting dark??

[evilmantis]

Good evening salty people!
I have noticed that the dorsal fin on my heniochus is rather dark, unlike when I first got him well over a month ago. It's been a gradual darkening. He eats well (eats like a pig, to be honest -- eating every pod he can reach and still begging for me to drop food in for him) and is very active. In fact, the activity sometimes concerns me -- he darts around and sometimes shakes. However, he does NOT scratch on any rocks. Now, there is a particular "cave" he favors which he has to back out of since it's too narrow even for him to turn around, so I'm wondering if part of the dark splotches are from injury in that respect.
Tank specs:
75g, been set up for about 2 years
75-100 lbs live rock
sand bed substrate
tankmates are a longspine urchin, a torch coral, and a mandarin (he's been there over a year, and is fat and happy)
refugium chock full of chaeto, pods, and whatever else is swimming in there
nitrates/nitrites/ammonia/phosphates all 0
salinity=1.025
temp=79
pH=8.0-8.2 (working on that)
Sound like any disease, or could it just be a combination of stress and injury? The temp did go up recently to 84 due to the lights (MH) being left on overnight when I left it in my husband's care.
Thanks!
Goldie

 

[nicetry]

It doesn't sound like an illness or injury. Many butterflies exhibit a variety of stress colorations. It may also be a natural darkening of the fin related to the fish maturing. If there are no other symptoms I wouldn't worry. You did mention the fish seems to shudder/shake every so often. How often is this happening? Healthy fish don't normally exhibit this behavior and I've seen it a lot with henis. Could be a parasite even w/o the fish glancing off rocks.