Ammonia level?
Alk is alright, but boosting it slightly to 10-11 dKH wouldn't hurt unless you're Carbon Dosing.
Making sure the Specific Gravity is accurate as dv3 stated wouldn't hurt either, lowering temp slightly to 78-80 would be beneficial at this altitude as well.
If Salinity is fluctuating between top offs, that may be contributing in a small system like this.
For feeding, again, as dv3 stated, typically not needed for Zoas, but if you like, Oyster Eggs, ground up Cyclopeeze, or simple Amino Acids and Carbs are often the best for zoanthids. Just be careful not to over do it.
When all else fails, changing the water never hurts as long as temp, SG, and PH are matched up ahead of time. The nice thing about Nano tanks is large scale water changes are pretty painless.
Didn't mean to come across as belittling, but we've got to have something to go on in order to trouble shoot!
