Aquariums: Almost all fish died in a day in 20 long

Is there any illness that could wipe out half of a tank of fish in a day? I have a heavily planted 20 long with tons of salvinia, a pothos, a philodendron, a powerhead, a small heater, and a sponge filter. About two weeks ago I got 6 Kuhli Loaches, 6 Royal Tetras, and a panda loach to add to my tank which had 4 neons, a german blue ram, and a hillstream loach. I’ve had fish die in the past but on my birthday a couple of days ago a ton of fish started dropping dead, 3 neons, the hillstream loach, 2 royals, and a kuhli loach. Most of them died within the same couple of hours, and my ammonia, nitrites, and nitrates all showed up as 0 and my pH was 7.7 which is what my tank always is. The temperature was a couple of degrees cooler than normal, but not anything crazy and its back up to 76 now. Ive been panicking and I did a water change immediately which I thought fixed the problem because stuff stopped dying and the fish looked good but now 3 days later and I found a two more dead royal tetras. I never spray anything in my room nor is there anything that could have fallen in, and the only “abnormal” thing is that i see a bit of cyano appearing around the same time but ive had full cyano takeovers before without any of my fish dying. Any advice? I’m very much panicking 😅

Note: my tank always looks gross im not sure why but its pretty much consistently detritus covered and i just vacuumed and cleaned it a few days ago (and had cleaning fish but well we know how that’s going), maybe because i replaced most of my old plants with new ones recently and they are still in a meting phase :/