Temperature matched — but the tank still crashed
This looks like: low KH (low carbonate hardness) letting pH swing or crash during water changes
Why this happens
KH is the buffer that stops daily acid creep. Tap with low KH mixed into a tank that drifted acidic can shock fish even at the same thermometer reading.
What to do right now
- Test KH (and GH) tomorrow morning — photo the strips or liquid readouts.
- Slow partial changes only until KH sits in a safe band for your stock (many community tanks like some KH).
- Crushed coral in a mesh bag or aragonite in the filter can raise KH slowly — go slow, re-test weekly.
- Avoid wild pH up/down potions — tiny daily stability beats chasing 7.0.
- If you keep soft-water species, research peat vs RO impact before buffering hard.
What not to do
- Don’t swing pH more than ~0.2 per day chasing a chart online.
Stop it happening again
Test KH occasionally; if it is very low, add a little crushed coral to buffer it, and change water in smaller, more frequent amounts to avoid pH swings.
If that doesn’t fix it
If KH looks fine but fish still die, screen for metals in rural well water.
Catch this earlier next time — log your water parameters in App-aquatic. Free forever.
This isn't veterinary advice - it's a directional read to help you decide your next step, based on the most common causes. We can't see your fish, so pair it with a liquid test kit and clear photos, and see an aquatic vet or a trusted local fish shop for a valuable or seriously ill fish.