Sherlock Holmes anyone ?

[Buccaneer]

Originally Posted by Van down by the river

When you were vacuuming did you pull up any smelly (blackish)spots aka: sulphur dioxide? It is a small possibility that you released this into the tank which would have caused the heavy breathing ( but so would released nematocysts).Your circulation would have dissipated it fairly quickly and the larger hardier fish may have been able to tolerate the diluted amounts.

.

I did see some blackish spots ( did not notice an odour though ) ... maybe because the smaller fish were hiding in the rockwork ( there is alot of rock ) and also the shrimp dont come out till after the brighter lights go out so they would have been in the rockwork as well

Originally Posted by Van down by the river

I think that the dramatic changing of sand and 30 gallon water change are most likely the causing culprit. I don't know the full size of your tank but 30gal water change of new water and the stirring of the sand bed could have caused low oxygen levels as well.
I think you will never really know which it was or maybe a combination of some or all of these possibilities.

The tank is 300G / sump 70G / Refugium 50G ... with sand and rocks the water volume is probably close to 300G so the water change should have been in the 10% range ... like you said a combination of factors may be involved here ... thanks for your response

Cheers