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).
a litle about the two types of bacteria in the nitrogen cycle:
the bacteria nitrosomonas is the one converting amonia to nitrite they grow slowly and this is the reason it takes some time to cycle a fish tank. https://en.wikipedia.org/wiki/Nitrosomonas.
the other bacteria nitrobacter multiply faster but they need nitrite to live so they starts to multiply when the nitrite is availiable in the water they produce the nitrate(plant food). https://en.wikipedia.org/wiki/Nitrobacter.
the two types of bacteria form a layer first nitrosomonas(they uses co2)and on top of them the nitrobacter grows(they use oxygen,o2). additional information can be found in the two links provided.