Hello, I recently got into keeping fish, with no experience at all before this. Two weeks ago I purchased Larry, who was 9 weeks old at the time. He appeared very healthy initially, with bright beautiful fins as seen in the second picture. He was shy at first, but ate and flared normally, but over time he became more comfortable with me and is able to jump higher and higher to feed off my finger. However, as you can see, he's been developing what I think is fin rot, and it seems to be getting worse every hour. Here's some background info to help diagnose and lead me to the right steps: I initially had no knowledge at all of the nitrogen cycle. I put him in an uncycled, unfiltered, and unheated 5 gallon tank I had lying around in my house from god knows how many years ago. Had him there for roughly a week, during which I was researching and realizing my mistakes.I now have him in a 5 gallon heated, filtered tank, and doing 20% water changes every 2 days to hopefully offset the ammonia. I haven't done a water test yet, but I'll be taking a sample to the pet store tomorrow to check (I can update with the numbers if it helps).Water temperature is currently sitting at 24 degrees C. Not happy at all with the heater I got (Marina C25 compact heater). It's supposed to automatically adjust to 26 degrees but it's stubborn at 24. Will be returning that and buying a new one hopefully.He seemed to get worse after I introduced the heater, thermometer and some Marimo moss, which arrived a few days late from Amazon. Not sure if it's related or coincidental.I used some bacteria cycle starter for the first 3 days in his new tank, in hopes that it would kickstart the Nitrogen cycle.Feeding him 3 flakes twice daily, seems to be happy with that.He recently started to get a little lazy, resting on various objects and hiding in his cave. But when he sees people, he comes out to greet them and swims around energetically and appears fine.So to summarize, his health appears decent in terms of activity and human interaction. His fins, on the other hand, indicate otherwise. Thank you for your help in advance!