Hello, I recently got a 36 gallon bowfront about 2 months ago, the guy gave me everything I needed except the filter but I had filters from my old tank that I kept running on my 75 to make sure that the bacteria wouldn't die. Anyways, I did a water change last night and everything seemed fine but all the mbunas passed except 1. I don't what it could be. Filters were cycled,water was dechlorinated, temperature was right, ph,nitrates,nitrites all were in good levels. What could have happened