can we see how your powerheads are set up? did you check the tds? or was the unit purchased new?
Something ain't right here.
Typically in all Maintence adventures After Hours...At friends houses, etc.
I Turn Off Tank Flo, Have Water Ready say Good 20%, 30% being on safe side.
Really simple, Airline Vacuum Thick Layer Above Gravel.
Slow, wins! Then try and suck off as much as possible. Pinch the airline, get a Huge Net, slightly place near major rock..get airline going in net..Gently lift rock into net not spreading it.
***Then lift out and put into old tankwater bucket just vacuumed...(Can net off chunks before and throw away.)***
I would then double dose the bucket. Try and shake up good in bad and add to rock... Then swirl around.
Now remove your carbon, don't touch your prefilter unless caked too.
Throw out carbon, rinse sponge in treated bucket if caked.
Now go back with 3/8-1/2 hose wo vacuum suck just surface of affected areas.( Try and do in separate bucket)
20 minutes, Put rock back in tank letting drip off near Complete...
Refill tank with quality new saltwater, start up filters.
Then I just throw out the sand of was that bad and not too much...
***Key****
Now dose the tank by 2x initial dose, then dose few days in a row normal total volume dosages to tank size...
By doing this trick, never had reoccurances.