By default a water change is the first thing I'd do, then run lots of carbon. Retest ammonia and nitrates. Make sure you give the test enough time for the colours to actually show up; most people don't do this and just go "Oh, ammonia and nitrates zero".
Do you have softies in your tank? Could be a bout of chemical warfare.
How's the temperature looking?
Is the tank getting enough aeration/gass exchange?
Did anything poisonous die?
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