My brother was watching my house for me for two days over thanksgiving vacation. I return home only to find my 10'' female reticulated ray dead. Did a 250 gallon water change on the tank. Tank is 400 gallon with 100 gallon sump. tested the water agian this morning with bad results. ammonia near between 1 and 2ppm, nitrite 0ppm nitrate around 40ppm. Is my tank starting a cycle or should I do more water changes untill levels have dropped to safe readings? i have a dart pump @3600gph media:filter- carbon- filter - bioball and then returned. Im also running an ehiem proffesional cannister and fx-5 on the tank. thanks for you advise.