Sounds similiar to what happened in my 55g tank only I would lose new fish at the 6 month point.I have a 55 gallon tank that is 6 months old. I was trying to raise the PH by adding baking soda (never again), and alll my fish died. I did a 20% water change and tried to add 2 new fish. Both died as well. Should i do a 50% water change?
By figuring out what is causing the decline in PH you can nip the problem in the bud. I found purchasing a product for the job of raising PH can be a better alternative as many will never raise the PH past 8.3 or 8.2 even if you fail to dose correctly.
I'm sorry didn't mean to come on too strong.FWIW I was stating that finding the initial problem is key to the solution. I have seen PH drops from many things from improper decor and sump materials to simple exhaustion of the water column from excessive coral growth. I hope you don't deem me Ignorant and Dangerous when I also make a recommendation for a future dosing product that I feel is a tad bit safer than baking soda,as I did not say to use it I was insinuating to look into it.
Also I don't see a statement about the tank still cycling but maybe that is in another thread. Sorry for the misinterpreted information.
So Kennie can you give any other information on your tank such as water perameters including PH and KH. Is this gentleman Beaslbob right and your tank is still cycling and suffering from carbonic acid buildup?
I must also say that say 5 of my reef tanks have a slightly lower PH and still have great coral growth,color, and beautiful fishes...the PH is usally 7.9ish so don't worry too much unless its really low and or fluxing.