By changing the gravel you have removed an overly 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 get 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 ammonia and nitrite on your fish.
Test the water daily and start feeding the fish slowly when the water is normal. At first, start by feeding only once a week and if the water stays good, you can slowly increase the frequency of feeding.
A little about the two types of bacteria in the nitrogen cycle:
The bacteria Nitrosomonas are the one converting ammonia to nitrite, they grow slowly and this is the reason it takes some time to cycle a fish tank. More details about Nitrosomonas could be found on Wikipedia.
The other bacteria Nitrobacter multiply faster, but they need nitrite to live so they start to multiply when the nitrite is availiable in the water. These bacteria produce the nitrate, which acts as fertilizer for the plants. More details about Nitrobacter could be found on Wikipedia.
These two types of bacteria form a layer, first Nitrosomonas (they use CO2) and on top of them the Nitrobacter grows (they use oxygen - O2).
Additional information can be found in the two links provided.
