Oh holy moly. That took all of 30 seconds to understand. I can't believe they don't print a chart like that and stick it in with the test kits.
By that standard my bettas have never ever endured ammonia above .002. Even when the pet store told me my ammonia was super super high and the reason why the snail or betta was getting sick. Turns out I never had anything living in toxic levels of ammonia, nitrite, or nitrate.
The other thing they said was pH swings, as it was going from 7 to 6 a lot. Rarely, down below 6.
But I'm beginning to think it was part pH, part using a dechlorinator with nothing to help slime coat, part total lack of minerals, part temperature swings. I just learned the heater in those tanks just raises 2 degrees over room temp, and that room was pretty darn cold at night through the winter. Their tank lights put off a lot of heat in daytime too. They were likely going through a 10 degree swing each day. (Yesterday Betta Bob got a heater with a thermostat that stays at 78).
Well that helps put it all in perspective. So how do I know when to change water? If I have .25 ammonia, 6.5 pH, 78 degrees, zero nitrite or nitrate, GH and KH about 3-5 and stable, and I haven't added anything to the water (increasing tds) ... In this case it's the betta tank ...