I can see now what the problems are. First and foremost, the tank is too new for that many fish. It takes as long as you've had them to fully establish a nitrogen cycle and should only be done with a very minimal amount of fish if any at all. You can cycle a tank with just some live rock or fish flake food.
I think the tank hasn't had the chance to establish a cycle because of the amount of water changed...which, BTW
, is too much at once for any marine unless there is a serious issue with water quality. 20% to 30% is too much of a sudden change. Marine animals are not adapted for such fluctuations in their environment like most freshwater fish are. This is where the difficulties of keeping marine are at...keeping the water stable. Though that much of a change will help keep ammonia from spiking, in order to establish a nitrogen cycle, it has to spike as well as nitrites. It's the nature of things when it comes to these little bio bacterias.
What I would suggest is for you to cut down the population. You can trade some out and get them back later if you so choose to. You could even trade them all out for six damsels you can return when the bio has finally established. Then you can go ahead and start populating the tank one fish at a time giving a two week rest in between to allow the new fish to establish territory and for the bio to catch up to the new waste load. Always introduce lesser aggressives first and check on adult sizes before purchasing an animal. Triggers should always be introduced last.
I would not put a trigger in with a damsel in a QT
. That's like putting a cat in a bird cage with the bird.
should be nothing but a bare bottom tank with a filter, heater, thermometer and a hide out for the fish. All new fish before introduction into the main system should be in this tank for at least a week, preferrably two. If any of the new fish get sick you can treat them right in the QT
. A mix of tank water and new water should be used doing a 10% water change once or twice a week.
I know it's hard to let go of the fish you have already aquired, but it would be best for the system to just start all over or keep the damsel in there and take it from there.
If you are absolutely reluctant to do that, then the best thing to do is 10% water changes two to three times a week until the nitrogen cycle has established. You can double dose bio additives like Nitromax Marine every other day until the cycle has established. Also use Prime or Amquel Plus water conditioners to help protect the fish against nitrite poisoning. Feed very sparingly during this time. Every two days what they can eat in a four minute time span for the day and nothing more. Maybe even a little less.
Test the water once to twice a week and monitor the levels. Ammonia will spike, then as the ammonia starts to go down, the nitrites will spike. When nitrites start to go down, nitrates will start to rise. Finally everything should settle with ammonia and nitrites zero and nitrates should be maintained at no more than 20ppm in a mairne system that is not reef. Basic routine water changes for an established tank should be about 15% every two weeks. With puffers, eels, lions, and triggers, that would require a 10% water change once a week given the fact that they produce more waste in the system.
Goodluck and keep us updated.