All fish died overnight

masonpep · 2026-06-30T11:51:16-0400

I woke up yesterday morning to find all my fish dead. But my pistol, shrimp cleaner, shrimp, hermit, crabs, and snails survived. Corals were stressed out too. Not too sure on how this would’ve happened, but here’s some context.
The night before I noticed that my Canary blenny was not doing well at all. He had a sunken stomach and was swimming very lethargic. And I knew he wasn’t gonna make it through the night. I tried to feed him with some flakes that I had he wouldn’t eat anything and then proceeded to do a water change. Didn’t realize that my auto top off system was going off shut it off as quick as I could, but then drained a little bit more water to counter the freshwater that was dumped in. Filled everything (un heated water) up and went to bed the next morning I woke up and all of my fish were dead. I wasn’t able to test until around 4 o’clock cause I had to go to work, but by the time that I tested, everything looked fine. 35 PPT salinity zero ammonia zero nitrite 20 nitrates 8.0 PH in 7DKH. Anybody know what could’ve possibly happened?

nautical_nathaniel · 2026-06-30T11:57:15-0400

No puffers or boxfish, right?

masonpep · 2026-06-30T12:03:36-0400

nautical_nathaniel said:
No puffers or boxfish, right?

No, I had two clowns a canary blenny, watchmen goby, and a tailspot blenny. In a 28 gallon nano cube. Running live rock and bio rings

Kooma · 2026-06-30T12:05:50-0400

Water was clear or cloudy?

Uncle99 · 2026-06-30T12:06:32-0400

Shock from increasing salinity to fast?
Is that possible when you corrected the ATO?

masonpep · 2026-06-30T12:40:14-0400

Kooma said:
Water was clear or cloudy?

Clear water

masonpep · 2026-06-30T12:40:57-0400

Uncle99 said:
Shock from increasing salinity to fast?
Is that possible when you corrected the ATO?

That’s way google says I think I could be a mix of salinity and temp shock but it just seems strange

Fish Fan · 2026-06-30T12:58:14-0400

You can safely lower salinity quickly, only increasing it quickly is dangerous.

masonpep said:
Filled everything (un heated water) up and went to bed the next morning I woke up and all of my fish were dead.

Was it maybe temperature shock?

Uncle99 · 2026-06-30T18:22:25-0400

masonpep said:
That’s way google says I think I could be a mix of salinity and temp shock but it just seems strange

It would depend on how far down ATO took salinity to, how long it remained that way, and how fast, it was returned to 35ppt. If it was simply down for a few minutes and returned, I’d rule that out.