Sounds like a bacterial bloom.
Something must have disturbed the beneficial bacteria (
The Nitrogen Cycle)...
If your tank is not newly set up and was doing good before, I would suspect that over cleaning the filter and preforming large water changes caused too much good bacteria to be washed away or removed from the tank.
Bacterial blooms cloud the water and are unsightly, however they pose no harm to the fish.
The fish are only in danger if you have ammonia and nitrites present in the water.
It is possible that too much bacteria was removed and not enough remained to convert the toxins (ammonia and nitrite). This is a possible reason as to why your fish are gasping for air.
Do you have any means of testing your water?
It would be very helpful if I knew the readings for:
Ammonia
Nitrite
Nitrate
From your short description, I'm suspecting that the nitrogen cycle was disturbed (cloudy water) to a point that allowed the toxins ammonia and nitrite to build up high levels (cloudy water and ill fish).
This situation clears on it's own as the nitrogen cycle repairs itself.
It will produce massive amounts of good bacteria at a high rate.
The bacteria clouds the water because it has not yet colonized.
This is known as a bacterial bloom.
Once the bacteria colonize inside the filter and gravel,
The water will begin to clear.
If there is ammonia and nitrite present in the water, their levels will also return to 0 as the nitrogen cycle reestablishes itself.
The amount of time it will take for everything to return to normal will depend on how much bacteria needs to be replaced.
The cloudiness usually gets worse before it gets better, so don't be alarmed if this happens.
I had a bacterial bloom last 4 weeks once.
The best thing to do for your tank is to do nothing at all.
It will stabilize on it's own.
However, if your fish are showing signs of distress,
I'd recommend doing a 10% water change daily in order to dilute the toxins.
This step may not be necessary but only water tests can determine that.
Please let me know if you have any further questions, I'm here to help.
Kae