The background gave me a start, lol. Welcome to the forum! Your tank looks great! To answer your questions:
1. Get a thermometer, it will save you the headache; as well as any emergency temperature fluctuations that occur during your fishkeeping. They're like, $3 for the ones you stick INSIDE the tank. The sticker ones are not that accurate, and are difficult to read.
2. In that 10 gallon aquarium, is that a filter behind the heater? In any case, here is a schedule for both filtered and unfiltered 10 gallons:
If you leave it unfiltered: 50% twice a week with a 100% every other week.
If you buy a filter in the near future: 25% once a week if you just have a betta.
Having a filter will allow your tank to establish a natural ecosystem, which will aid you in removing harmful compounds from your fish's waste.
Also, as a side project, you can invest on getting your boy a hut or cave to go in, or some tall plants (silk preferred) for him to play and sleep in :). Your boy looks so big in that 10 gallon <33