SEACHEMS STABILITY STAT! Also they have ammonia removing media you can throw in the filter. Until then, water change water change water change. I know it's pretty common for people to say don't change more than 50% of the water at a time, but in this case I would get the ammonia out of there, drain the tank to just enough for them to swim in. (Like 10 gallons) then add 10, remove 10, add 10, remove 10, add ten remove ten. Do the math, (basically doubling the amount of water, cut it in half, double cut in half, each time you do that your cutting the ammonia in half. When it's close to zero, refill the tank (which will distill it even more, down to trace amounts.) Add a big dose, maybe more than recommended of Seachems Stability, and follow the instructions, I.e. Turning off UV
filter. I'm sure others will condemn this, because it will stress the fish, but let's be honest, the fish are already stressed. Get the ammonia out of there, check it often, use stability, and be ready to go through a similar battle with Nitrites. Good luck, but seriously: Stability.