My Angelfish has redness at the base of his pectoral fin. I recently introduced 4 blue rams. One of them died. My tank also had other three angelfish, the partner of the one in the picture and two smaller ones that died yesterday. On my tank I also have 8 tetras, and a common pleco. What is causing this redness? It had happened once before and I did a water change and he got better. Yesterday I did a 50% water change. What should I look out for? or what can I do?
tanks size:75 gallon
ammonia: 0 ppm
nitrite: 0 ppm
nitrate: 20 ppm
pH: 7.6
temp: 80 F