The filters are a fluval 305 and ehiem 2227.Around 40 kilos of live rock.I have had this setup for 8 years with no previous problems.The tank holds around 100 gallons.Ammonia 0,nitrite 0,nitrate 40ppm,phosphate 0,s.g 1.023,temp 75-79 degrees.I presume the existing fish have built up a resistance...