All fish died overnight

[masonpep]

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]

No puffers or boxfish, right?

 

[masonpep]

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]

Water was clear or cloudy?

 

[Uncle99]

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

 

[masonpep]

Water was clear or cloudy?

Clear water

 

[masonpep]

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]

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

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]

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.

 

[masonpep]

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.

Guy at the fish store says it could have been velvet. Idk though because my tank has been settled for around 2 months

 

[strich]

I doubt velvet killed all the fish that quickly on its own. But maybe velvet kicked things off, weakened the fish and reduced their oxygen intake. Then, unfortunately, you may have put the final nail in the coffin by dumping cold water in causing temperature shock.

 

[Fish Fan]

Guy at the fish store says it could have been velvet. Idk though because my tank has been settled for around 2 months

Marine Velvet is somewhat rare, but lethal enough to kill all your fish in a very short period of time, like 12-36 hours or so. Here's a post from Jay Hemdal (our resident expert in fish diseases) on the symptoms of Velvet, did you notice anything like this?

https://www.reef2reef.com/threads/velvet-outbreak-help.1157286/post-14330907

If there's Velvet or another pathogen in the tank, you'd want to leave your tank fallow (fishless) for 45 to 60 days so that any fish parasite will die off without a fish host, before adding any new fish to the tank. It's also a really great idea to QT your own fish, or purchase pre-QT'd fish from one of the vendors providing this service.

 

[LeftyReefer]

I doubt velvet killed all the fish that quickly on its own. But maybe velvet kicked things off, weakened the fish and reduced their oxygen intake. Then, unfortunately, you may have put the final nail in the coffin by dumping cold water in causing temperature shock.

I'm going the other way. I think it was more likely disease that killed all the fish or combination with a low oxygen event, more so than cold water shock. Tank is an AIO and OP said it's running live rock and bio-rings.... which means he's probably not running any skimmer and most AIO's are cubes and don't have great oxygen exchange to begin with and combine that with a heat wave.... low oxygen.

I do water changes with unheated water all the time on my tanks and my fish have never seemed to care. In fact, I've never ever pre-heated water change water. my fish will regularly see 5 degree temp swings during a large water change and it doesn't affect them at all. My water change station is in my basement. unheated water in my basement stays around 60F all year long.

When I do a normal weekly 10%-15% water change, my temps may drop a degree or two, but when I do a big water change, temps can swing 5-6 degrees and I've never had any issues with anything, inverts, corals, or fish.

I just did a 50% water change on a 45 gallon tank the other day using unheated water pumped up from my basement. The temp in that tank dropped from 77F to under 72F while I was refilling it with the 60F water from my basement. None of the fish in the tank were affected at all and they all went about their business like nothing was wrong. I fed the tank as soon as I was done and everyone was eating and happy, unaffected by a 5+ degree temp swing.

I had a magnet go bad in one of my tanks a while back and it wiped out just about all of my corals. I had to do a bunch of big water changes in that tank to get the heavy metals out and none of my fish ever seemed bothered by any big water changes with colder water.

Healthy fish can withstand temp and salinity and oxygen swings better than sick fish.
I think OP had sick fish and a low oxygen event took them out.
OP even said some fish were showing signs of trouble before the water change happened.
Tank is only 2 months old. Sick and weak fish from the start is likely the cause and maybe oxygen levels dipped during the night enough to kill the fish.

Unless the tank dropped below 70F, I doubt water change water temp had anything to do with it.
and you can pretty clearly tell when a fish is stunned by cold water. I think OP would have noticed an issue with the fish right away if temp was the issue. Cold fish don't behave normally, but sick fish can, right up till it's too late.

I think it was sick fish and a low oxygen event took them out.
probably not a coincidence that much of the world is currently seeing a heat wave right now too, I sincerely doubt a water change with cool water took these fish out unless there was an underlying issue.
elevated tank temps and low oxygen and disease are more likely to have killed these fish than a water change with unheated (cool) water. Unheated water is typically room temperature, so it's not like it's truly COLD anyway.

 

[BryanM]

Can we get more info on how the tank is set up? return pump, wave makers, maybe a video showing the surface of the water?

 

[masonpep]

I doubt velvet killed all the fish that quickly on its own. But maybe velvet kicked things off, weakened the fish and reduced their oxygen intake. Then, unfortunately, you may have put the final nail in the coffin by dumping cold water in causing temperature shock.

Do you know how I can be sure it was or was not velvet I would love to add fish sooner then later and I read if it was velvet I would have to wait 3 month before adding fish

 

[Fish Fan]

The fallow period for Velvet and Brook are shorter than for Ich. Jay Hemdal recommends as little as 45 days fallow for Velvet, but 60 for Ich. And, if you don't really know what you had (though I think you'd know if you had Ich), it's best practice to fallow for 60 days. But not three months

Here's some quotes from Jay on this:

https://www.reef2reef.com/threads/tank-crash-due-to-marine-velvet-how-to-feed-lps-and-crabs.1152229/post-14292423

https://www.reef2reef.com/threads/velvet-or-ich.1154122/post-14279979

And again, I would strongly consider QT'ing your own fish per this guide. It's easy, and you can do this while you're fallowing your display tank, so you can get new fish right away. Or, purchase fish from a vendor that does a full and proper QT paradigm. Most fish stores do not do a full, medicated QT even if they say their fish are QT'd, it usually just means a week or two of observation:
https://www.reef2reef.com/threads/current-quarantine-protocol.825055/

I hope that helps!