if i remember correctly you can get a build up of hydrogen sulphide (sulphate? note entirely sure dw is around here somewhere he'll know the exact gas he knows everything
) in the substrate from rotting food waste and poop etc.
this when disturbed and released into the water column is highly toxic to the fishies if you've got a deep gravel bed but only disturb the top few layers when vaccing you could have a build up further down or you've released a build up etc.
take the fishies out give the gravel a good mix up drag up everything from the bottom to the top etc really make a mess. drain the tank down and suck out all the stuff you've just dragged up from the bottom (believe me there'll be LOTS) refil allow to settle and you'll be good to go again.
this when disturbed and released into the water column is highly toxic to the fishies if you've got a deep gravel bed but only disturb the top few layers when vaccing you could have a build up further down or you've released a build up etc.
take the fishies out give the gravel a good mix up drag up everything from the bottom to the top etc really make a mess. drain the tank down and suck out all the stuff you've just dragged up from the bottom (believe me there'll be LOTS) refil allow to settle and you'll be good to go again.