by changing the gravel you have removed a too large portion of the bacteria.
stop feeding and angle the water outlet from your pump/filter upwards to get more movement of water in the surface(to ger more oxygen in the water).
use some water from one of your other tanks when you change water if this is possible to do.
medicating the whole tank will kill the bacteria so it is best to avoid this.
add some aquarium salt to reduce the effects of amonia and nitrite on your fish. test the water daily and start feeding the fish slowly when the water is normal(in the start only once a week and if the water stay good you can slowly increase the feeding).