Cory cat trouble

[Chris OB]

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!! 😢

[xXInkedPhoenixX]

That is really strange @Chris OB. I'm wondering you didn't mention other parameters like kh/gh do you test for that? also ph. And if you do test for (all of) those is there a difference between the tank they were raised in and the tank they're in now? What is different between that tank and your others? (substrate, decorations, plants, filtration, anything that might give a clue) Temperatures between the two tanks?

[Chris OB]

All parameters are the same, same substrate plants etc, kh-7 gh-9 ph 7.2

across all the tanks, this one tank they just go lethargic and die off. It’s frustrating and upsetting because it’s happened twice now in a year basically. All the other fish in the tank sift the same substrate and in my other tank I have 10 very happy healthy Cory’s . I’m going crazy