I have a 30G about a year old... Got a mandarin about 10 weeks ago, still looks good and is pretty active. I'm I in the clear? How long does it take to starve one? Disclaimer, I've added pods every month as a safety net.
If you're the DIY type, lots of videos on YouTube on how to make a dual temp controller. Search for inkbird it cost me about $40 total, controls heater and fan. Otherwise I recommend buying a used reefkeeper, a little more than the ranco but with lots more options.