I recently had something similar with my 500lt Rift Lake dont know the contamination was but one dead fish + contaminated bucket = lots of water changes I also used Nitra-Zorb and StressZyme Filtration Booster to speed up recovery. Then last weekend all the fish, cichlids, bn's , and syno's gasping on surface babies everywhere and nothing getting eaten,,, massive ammonia spike , so more water changes 100lts one day then 50 the next,, everything back to normal now. Hard to sleep the night through now , without getting up to check on everythig! Hope it all goes well for your setup from now on.