DaYooper_Duck
Songster
The inch per gallon rule is highly outdated and inaccurate. No fish can happily live in anything under 3 gallons. Just because a human can live his whole life in a tiny, filthy jail cell doesn't mean he should. Futhermore, there are one-inch fish that produce significantly more waste than a different same-sized species, like the guppy fish, and need about 2.5 gallons per fish.The rule of thumb for tropical fish and bettas is one gallon per inch of fish. Since bettas are a bit tall they need a bit more so 1.5-2 gallons is a good amount for a betta even with a filter IMO - it's a huge step up from the "Betta cups" they're sold in, and even in a 10gal they don't move around a ton. Mine was in a 10 gallon and just frankly didn't utilize it. He liked to hide in about 3 spots and that was it. With a 10 gallon you only need to do a moderate (%25-%50) water change every month, whereas a smaller tank would need it weekly. More fish = more water changes.
Also, bettas are very active fish by nature, and love to explore every corner and nook and cranny of their tank, so there was likely something wrong with your water parameters to cause your fish to be lethargic.
Finally, water changes should be carried out weekly, not monthly, to avoid nitrate buildup in the water.