Threadfin rainbows suddenly died?

I have a 75g relatively new relatively empty tank. However, the filter media is 10 years old so it was almost instantly cycled.. that said, I waited a while ensuring it was fully cycled before adding fish (fed it ammonia, tested carefully, did water changes to keep nitrate barely detectable). Everything is spot on.

I had a group of 6 threadfin rainbows, 6 cories, and 10 cardinal tetras.. plus a few plants. Everything is (was) doing well.

The threadfin rainbows seemed to be doing well.. a little concerned they weren’t eating because their mouths are so tiny but found crushing flake food worked. They seemed to be happy.. males were flashing, color was good, active, not hiding. Had them for maybe 2 weeks.

Within 24 hrs the entire group died. No other fish affected. No sign of disease or anything. The very last one that died was the only one I saw acting weird - it was circling at the surface. So I immediately thought an oxygen issue? But I have pretty good surface agitation and added an air stone. Died within an hour of seeing him like that. Any ideas?