So I want to start off with I am not new to the hobby. Been in it for 30 years and worked at a specialty fish store for some time. I have been stumped by my mass die out of African cichlids mostly my peacocks. There are no visible signs that would suggest any reason for the deaths. The fish have all been in the aquarium for over a year and a half, some 2 years. My ammonia is zero, my nitrite is 0, my nitrate is 0, my ph is 7.0, and the temperature is around 76°f. The tank is a 75 gallon running two wall hanger aqaueon filters meant for 75 gallon tanks (I like the extra filtration and only run charcoal in one filter). I change the water every week and sometimes every 4 days. In the past 3 weeks I have lost 3 of my wild caught s1s (my wallet about cried), a sunshine peacock, my red empress, my super red, and my electric blue ahli. No signs or symptoms of anything. I treated with 2 rounds of melafix and pimafix. I'm down to 1 s1 wildcaught blueberry ob, 3 clown loaches, and a flash pleco. The loaches and pleco seem completely fine. Any ideas on what is causing my mass die out? I'm at a loss as to what is happening. I do run a sand floor in the tank but I rake the sand and am 100% I have no ammonia bubbles happening. Help me out here guys, trying to atleast save my blueberry.