nehaali

Hi guys, I have just installed a metal check valve to my return pumps so that I can use them as external pump. However, I have lost 1 star fish, yellow tang, blue tang and fox face since then. My 3 RBTA are not doing well suddenly now. My clownfish, trigger, chromis, Mandarin, Koran are looking healthy. I have checked ammonia nitrite and nitrate. All are normal like past 4mths. I suspect that the check valve might be copper and it is releasing copper into the water. Is this possible?