Nitrate

this will only help so far IMO, as mentioned angels have attitude!! in a small system such as yours your a bound to trouble, im surprised ich hasnt showed up yet. as far as i know angels are susceptible to this parasite when stressed due to poor water quality or bullying due to overstocking.

as mentioned if you have a good salt mix you do not need to add any further trace elements as they will all be present in the salt (personally i use coral pro by red sea and im damn happy with them).

keep up with your water changes thats a very very good habit. as mentioned by others i would remove some critters for everyones sake.

 

Your absolutely right. It only solves half the problem.