I have the API Master Test Kit, which doesn’t read kh or gh.
My substrate is Caribsea Supernaturals Aquarium Sand, but I don’t think it’s the problem, because my other tank has the same sand, with Cory cats and a bristlenose pleco.
Now he has Popeye, and he seems bloated and his sides seem to have “bubbles” in them.
Your nitrites are the problem, far too high, need to be 0.
Do an immediate 50-75% water change and get some Seachem Prime to reduce the effects of the nitrite poisoning. Dose the Prime with your water changes, do water changes every other day until that nitrite reads 0 and stays 0.
I would still.figure out what happened. Temminikii are usually pretty damn hardy fish. They can survive in a lot of.environments, so, there is somethin goin on in that tank. For the sake of the other fish, do a large water change and get them.levels down.
Sorry for your loss. Do a 100% water change to bring your nitrite level to 0. Go easy on the feeding, perhaps feed them every 2 days as opposed to each day until you can get persistent readings of 0ppm for ammonia and nitrite.