OK, Anemone's need perfect water. If you have Nitrite and Ammonia showing, there is your gaping mouth issue causer. There isn't enough light there, even for a BTA, and you still haven't told us about flow or tankmates... You need to get a large (50-75%) water change done, five minuites ago, to get the Nitrite and Ammonia down. You then need to work out why they are raised in the first place, so you can keep them down. Nitrite isn't overly toxic to fish, but highly toxic to invertibrates. Ammonia is poisonous to all livestock. For this reason, with invertibrates in the tank like Anemone's, they
need to be zero. If your Nitrite isn't zero, it
is high enough to cause grave concern. Having even trace amounts of either showing can kill invertibrates very quickly.
I'm sure this isn't what you want to hear, but your best plan of action from here is to remove the Anemone from your system, and to get it moved to someone with the ability to care for it correctly (i.e. have a year old tank with 4 T5 lights over it, minimum 20X an hour tank turn-over, zero's on Ammonia and Nitrite, with nitrate below 10, guards on all pumps and fitler inlets e.t.c). In a tank that new, with dodgy water parameters, poor lights and with an (no offence intended here, sorry if it sounds harsh) in-experienced marine keeper, the only way that Anemone's going is down-hill

Anemone's remain one of a very few marine invertibrates that are considered difficult to keep alive, even in ideal conditions, and really are best left in the Ocean, or with experienced fish-keepers only
All the best
Rabbut