Thank you guys!
Man, I feel really bad that the high nitrites did this
I've worked solidly to do large changes every day, but I did think I should be doing it twice daily instead, there just wasn't time between everything else, and the fish and shrimp didn't seem to be reacting to it the way I expected they would if it was building up and bothering them, no gasping or staying high or low.
It's okay, I'm not giving up guppies because I'm devastated and feel disheartened, it was always the plan to wind down keeping guppies and move onto keeping something else instead. Not just because they're poorly bred and mass produced and much weaker, but because breeding them is a lot of work, spare tanks, sorting fry, taking fry to the store... I've done it for almost 18 months now, and I'd already reduced the numbers of females as the original females died off, and I kept fewer youngsters. She was the last adult female I'd decided for sure to keep, so now that she's passed, I'd have to decide to keep some fry and continue breeding, or to raise them then send to LFS and get something else to keep. I've been tempted by psuedomuglis for a while, or I might get a larger tank and go for some dwarf neon rainbows or something.
Not giving up fishkeeping! Just going to move on from guppies I think. Doesn't mean I'll never keep them again! Might try some nice fancy guppies sometime. But ready to stop with livebearers at least for now I think other than the mollies/platies in the community tank. That's enough fry to sort already!