How to fix an ammonia spike in an established aquarium

Hello,

My tank has been up and running for a year without major problems--its a 20 gallon w/ sand substrate and a Fluval three media filter (sponge, carbon, and a bacteria holding media). Just one axolotl. Temp consistently at 18.5-20 degrees C.

Recently, I had my axolotl out of the tank for a week fridging due to fungus. During that time I did a 50% water change and replaced all the filter media (probably should not have done this) in case there was a water issue causing the fungus.

She's been back in the tank for about a week, and the water has gotten very cloudy. Ammonia is at one, nitrite at 0.25, nitrates are high. I immediately did a 50 % water change and added some API Quickstart live bacteria.

My question is--how do you deal w/ ammonia spikes in already established aquariums. I don't mind daily water changes until things settle down--but how much water should I take out? My worry is if I do too many big water changes, it won't give the tank enough time to build back up beneficial bacteria populations (since i'll be taking out growing bacteria with all the water changes). Maybe I'm just grossly misunderstanding the process. Also, how do I know when the tank if safe and fully cycled?

Beneficial bacteria - in their active form - live as part of a biofilm on the surfaces inside the tank. So changing out 50% of the water won't remove them.

Any idea what material caused the spike? It's usually caused by something decaying. Be super careful to remove all uneaten food and any obvious poo, especially until the water chemistry gets back to normal.

Also, do you know if your tap water contains chlorine vs. chloramine? If the latter, then large water changes will bring in ammonia with the water.

Thank you so much for your input!

From research on my city's purification process, it looks like chlorine. But that's a good point--I never thought to check the ammonia level of my tap water before adding it to the tank. I'm guessing the tap water should have zero ammonia (assuming no chloramines).

I'm not sure what caused the spike. A few weeks ago, I removed old food that was buried in the sand--the nitrites were spiking then--but after several water changes I had zero nitrites and ammonia so I thought the tank was fully cycled again. Is it possible to temporarily have zero ammonia and nitrite in a uncycled tank?

I'm keeping up with the water changes for now, and also added some API Ammo Block just to be safe. To double check, I'll know the danger has passed when I have zero ammonia and nitrite correct? Right now, it's my understanding that a cycled tank should always have zero ammonia and zero nitrite, the only changing parameter being the nitrates which would increase and then decrease after a water change.