I need some help, I have a 50 gallon tank that’s been running for a year now. All parameters are good, ammonia stays consistent at 0 same with nitrite, nitrate levels are at 20ppm after 7 days, I do weekly 40- 50% water changes . There are 5 neon rainbow fish and 4 maccullochi rainbows, 1 electric blue acara juvenile a peacock gudgeon and a pearl gourami. All perfectly healthy for months. Any time I’ve added my cory cats they start to die off with no overt symptoms. The tank has been treated with prazipro and ich x in the past. I even waited about 8 months after my last incident and added 3 cats about 5 days ago that I raised from babies from my 40 breeder , same water substrate plants parameters etc and I have 10 perfectly healthy Cory’s there, my quarantine tank even housed the 3 I moved to the 50 for almost a year. I’m frustrated and don’t want to move my super red bristlenose or add any bottom dwellers now. I just don’t get it!! 😢