ok so i have had this tank set up for several months (am very experienced with cichlids but have mostly kept Ca/Sa) with no issues. then yesterday i completely rearrange the tank and add in a slate cave wall to the back and now today everyone is dead :/ tank is a 55g with 19 mbuna and 1 peacock around 3" and under and a little syno cat (one jacobfreibergi male, no aggression issues he was one of the biggest in the tank so only 2 fish were dominant to him) parameters are zero nitrite, less than 5 nitrate, and .25 ammonia (almost certain this is only because i tested after i got home from work today instead of this morning and there were still several dead fish in the tank while i was at work as they were still alive when i left this morning :/) only things i can think of are that either somehow some of the silicon on the wall wasn't cured all the way (have been slowly working on it for months but newest last parts of the wall were done 3 days before adding to tank, figured this would be adequate as it was thin layers for the most part) no mold inhibitors or anything in the silicon its the same silicon that i used to put together the free-standing slate structure that has been in there since the tank was cycling with no problems, or possibly i stirred some kind of chemical reaction into the water when i was moving the sand about (though i have never experienced this since didnt think it was a problem with cichlid tanks since they are generally constantly moving the sand around and its not just laying there stagnant. when i got up this morning and saw that a lot of them were dead and the others looking like they were having issues breathing i hurried and did another water change and scooped out the dead ones and only filled it back up to 3in below the top of the tank so the filter could oxygenate more and added a water pump to further disturb the surface on the other side of the tank but the rest of them still died while i was at work :/ other than those two possibilities i am at a complete loss for why this has happened so any ideas would be greatly appreciated (also filter is an aquaclear 110 with tons of biomedia in it, def not a filtration issue)