Perhaps my previous comment was unclear, I did not say to never change filter media, I said don't change all the media at once. And rinsing in old tank water is something you can do on a more frequent basis than replacing. For example I had a 60 gallon tank with a HOB, every 5 days or do I would do a 30-40% water change, and take the main poop filter and slosh it around in a bucket of the water I was removing from the tank, and put the filter media back, this is because it was dechlorinated already rather than using water from the tap that would kill off any BB in it, this is a common practice, if you have a ready source of dechlorinated water to rinse the filter in, then there's no need to do this in old tank water. And you do what you want with your underwear. Then every 6 weeks or 2 months I would throw away the main filter cartridge and replace with a new one. I never replaced the other media in the filter, some ceramic media and a sponge just before it. These only got rinsed in water change water every month or so. Removing the poo from the first filter every few days kept the tank cleaner than waiting until it was time to throw it away and replace it.
Back to this particular tank, any ammonia reading is bad for the fish, and sometimes it's hard to pinpoint why it happened, but if the tanks been running for over a year, it should have been cycled by this point, so something happened to change that. As mentioned before by skjl47 it could have been meds, it could have been adding too many fish too fast, it could have been filter maintenance, who really knows, but you need to reestablish healthy levels of bb. Without magic bottles, the ammonia eating bb establish first and the ammonia levels decrease over time, a few days to a couple weeks, as the colony growsand processes the ammonia, they produce nitrite, so you would start seeing nitrite readings, and the second type of bb that eat nitrite will start colonizing and over time the nitrite readings will peak and decrease as the colony balances to the available foor source, and they produce nitrate. Once you styart seeing nitrate then you have both colonies started working, and over time if the tank population is stable and feedings are regular etc, they will balance to consume and process the ammonia and nitrite so effectively it reads 0 on your tests and youll only see nitrate. This is an established mature cycle. With a healthy mature cycle, if you add a lot of fish, or throw away 1/2 your filter media, you may see a blip of ammonia or nitrite, but it would be gone in a day, there's plenty of healthy bb to multiply quickly.
I'm not familiar enough with all the bottles available to tell you how they affect this process. some are starter bacteria that kickstart the process, others I believe react with the ammonia or nitrite making them less toxic or nuetralizing them to protect your fish, but don't boost your bb colony in any way. I would keep doing water changes every 2-3 days to reduce the amount of ammonia and nitrite in the water until the cycle is balanced. feed very lightly during this process, remove any uneaten food. Don't add any fish until it's cycled again. check your parameters on your qt tank as well, if thats reading 0 ammonia and 0 nitrite then it may be cycled and you could take a sponge from the filter, or some decor or something to put in the main tank and boost the cycle, if you've qtd long enough to be comfortable the fish aren't harboring any parasites or disease.
