Dont get me wrong I agree with everything posted so far, I agree your all I a bit high, I definitely agree to stop those nasty pellets! But when you say it's only fish, and corals aren't affected - that shoul narrow it down.
It's not about a protein skimmer, although I am of the opinion that you should have one, I don't think it would take out fish that quickly.
Mag levels, as mentioned are for corals.
So lemme try a different approach.
1. How are you oxygenating your water?
2. What are you using to test your "salinity"
3. Are you adding ANY liquids into your tank for any reason at all- if ao what and how much.
4. Have you "double checked" your readings with any othe source, like taken a sample to a LFS (local fish store)
Well, as I mentioned above, I narrowed it down to it being a sick fish on 2 separate occasions, not water quality, however I really appreciate your response. I oxegenate my water with 2 powerheads and my sump pump, I have over the reccomended 17x turnover (i think the reccomended is 17, but eitherway, its a little more. I actually dont check salinity, cause I dont really know the difference between that and sg, but I do check my sg with a refractometer. My sg is 1.024, which on the next few water changes ill be bringimg down a tad cause I like to have a little wiggle room for evaporation. The only liquids I have ever put in the tank are oyster feast food and pimafix & melafix (pima and mela were used just this week to cure the disease). I did double check my readings with my own test kits. I have api and tetra liquid kits, but I also took a sample to the store when my fish died. All results were in the safe range (except kh, which is being lowered gradually)