I’ve started a 55 gallon mbuna aquarium, with 20 fish of various species between 1 and 2 inches. The tank was fully cycled before I’ve added fish and I’ve had no problems with ammonia, nitrites (both zero), and nitrates have never exceeded 20 ppm. The pH is 7.6. I do a 50% water change once a week. My fish were alive healthy and eating for 3 months, but in the last 3 weeks 6 have died. I have not added any new fish or changed their diet. They eat spirulina flakes, nori, and cucumbers or zucchini. I haven’t had any aggression issues, and the fish that have died have fully intact fins. I don’t notice any spots or discoloration. All of the other fish seem healthy and are behaving normally. One by one a fish becomes lethargic and stops eating, and dies within a few days. I have tried treating the tank with super ick cure, melafix, and pimafix. Nothing seems to be working. Any suggestions?