Hoplisoma habrosum formerly Corydoras habrosus are rapidly dying off and we don’t know why.

Hello, in the last 48 hours we have lost 3(might soon be 4) of our originally 6 member school of habrosas. We cannot figure out what is going on so I am coming to you. We have this species in multiple tanks and have never had this happen.

Background:

This tank is a 10gallon that’s been running for 3 years.

Tested every month, no spikes previously (I’ll get to that). 25% water change biweekly.

Substrate is fluval aqua soil

Filter is a 3 type media also from fluval

Decor is a mix of driftwood, smooth stones, various seed pods to create lots of hides.

Plants: red tiger lilly, pearl weed, baby tears, Staurogyne Repens, and Rotala H’ra. There was at one point a pond lilly started from seed but it’s been moved to the outdoor pond.

The livestock in this tank were: the 6 habrosas, 1 khuli loach, cherry shrimp, ramshorn snails(I feed them regularly to my skink), 3 nerites and 1 mystery snail. (There was a female betta but we moved her to not upset the cory’s)

The 6 habrosas were added in 5 months ago. They were picked as breeding pairs from what we already had to get more genetic diversity. They had been doing great. We rotate feeding algae wafers, monster pellets, frozen bloodworms and freeze dried brine shrimp. They were nice and fat.

Now here is the chain of events. Last week the mystery snail suddenly died, we immediately removed it from the tank, no one seemed in distress. I check on the fish daily. Then this week on Tuesday morning I noticed that 3 of the corys were rapidly breathing, I checked the water. We had our first ever spike, ammonia 0, nitrite 0.25 and nitrate 80ppm. I immediately went out and got De-nitrate and switched that for the charcoal and did a 75% water change. Our parameters are now fine. But the catfish continue to breath rapidly and are dying. I started them on Paracleanse and an antibiotic just to be safe, but we have no idea what could be happening. All of the tanks the cats came out off are doing great and no one is in distress. And the loach in with them seems completely fine. Any ideas?