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.
How sure are we? Moderate — confirm with GH/KH tests; soft-water veterans see this often.
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.
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.