You dont have to fast them. I dont fast mine, but I also dont feed much food at once. I usually give my fish small, spaced out meals within a day. I give 3-4 micro pellets every 4-6 hours, generally speaking.
The fish might not grow that much, unless its a fry or juvenile.. if it is, the better way to get it to grow is through frequent water changes, as they produce something into the water that will keep them small.
Making sure, too, that there is at least 42% protein in the food will help make the fish healthy and strong.
How do you make a lid for a tank? I found a tank (An 8 gallon!! So excited!!) but it doesn't have a top. I read somewhere (Which could easily have been on this site) that you can use plastic wrap with holes poked into it, is that a good one to make?
Some will say every 2 days, but I disagree. I had a 1.5 and tested the ammonia, and the 50/100 a week is fine.
Dont let it go over that, though.
Even if filtered, in such a small tank, I do 100% changes in the week, even though with a filter you dont need to.
ive done 1 50->75 and a 1 100 per every 4 days before (1g tank). i found that the ammonia does get to 1ppm by the end of the 4th day after a 50wc 2 days ago. will the sudden drop/change in ppm bother the betta? i usually dont like the ammonia going over 1 ppm.