Ugh for some unknown reason my cycled goldfish tank decided to crash. I noticed my goldfish were being unusually lethargic and were breathing quite heavily, but as temperatures have been high here I thought it was because of that.
This tank runs four internal filters chock full of media and has been stable for three or so years now so I didn't think to check my parameters until my biggest goldfish showed some red marks on his tail.
Ammonia level was at like 4ppm! pH is over 7 so I was freaking out. In the course of two days I removed around 150L from the tank and overdosed on Prime to hopefully give them (and me) some breathing space. As soon as I added the Prime (I had been using API stress coat previously as I was running low on Prime and needed to conserve it for my other tanks), they immediately perked up and started swimming around.
Luckily after a couple of very thorough gravel cleanings with my siphon and some rinsing out of the filter media in each filter the ammonia problem seems to have cleared up overnight. Going to keep my eye on things for the next couple of weeks but thankfully my poor arm and back can get a reprieve from lugging around 20L buckets full of water.
Does make you realise though just how much ammonia goldfish are capable of producing in an uncycled environment. If it had been any other fish I reckon they would have been dead or extremely sick but my goldfish have returned completely to normal.