This happened about 2 months ago, but just getting on here to see if anyone has had this happen to them before. Here's what happened, had (6) 4-6" Indos with (2) 6" Severums, and (1) 5-6" Normal Pleco in a 225 gallon tank. All normal with fish and weekly WC with 2 Rena XP4s full of ceramics for 6-8 months and I come home to 5 out of the 6 dead! So within 12 hours from feeding them they died. The last Indo and other fish lived and are fine to this day. These were all with me since 1-2" and no new fish were added. The nitrate was high on the strip but no signs before of breathing heavy or anything out of the ordinary. Fish were healthy. The reason I am asking is because I moved my other 4 Dats to the tank and are getting to the same size and don't want to go through this again. Any input is appreciated.