Hi - I suspect a lot of us are getting unexpected and sudden nitrite and ammonia peaks, due to the heat. High temperatures make bacteria grow like crazy, as well as fungal spores, so excess food could well be spoiling fast. If you've had any fish losses (which I know a lot of us have had too, as in indirect result of the heatwave, plus some people have had power outages), they could also bump up the bacteria count before the bodies were retrieved.
Judging from the algae bloom I had in my tanks when I got back off holiday, plus the dead fish and plants, I reckon I had an ammonia surge too, and I was detecting nitrite the first day I came back. My tanks were mature and had had stable stock in them for some months or years so it was definitely environmental problems (plus some over-feeding from my neighbour) that lead to the problem.
I didn't think I'd ever say this, but now I'm praying for autumn!