Thing is.. filtered water isn't the best.. tap water is with conditioner. Filter water lacks the proper nutrients/minerals they need, that in over time, can cause them to become ill. Very highly recommended to use just tap water. And regardless the type of water, the ammonia will still build up rather quickly in a 1 gal. A 50% and a 100% water change per week is highly recommended to anything under 5g that isn't filtered, regardless of the type of water you are using.
Bubble blowing isn't just a sign of health for the most part.. it's instinct for territory, mating and even weather related when a system is moving in. I've seen many unhealthy, half dead bettas with bubble nests out there.
Fighters need the same care as others, so once a month isn't idea.
Yes, bettas can live a long time in unfavorable condition, even a couple years. But it does catch up to them, and it does take off years of their life.
Not saying you do not care for them, but that there is still the basic necessities that is needed, and it seems that you are missing a couple of them.
I don't care if your way of maintaining bettas is different, there's just no possible way that keeping them in 1 gallon jars without any heating and lined up to one another without proper water changes is "healthy". I've seen PetCo bettas with full bubblenests covering the top, then come in the next day half dead. Bubblenests does not, in any way, mean "very healthy bettas"!!
This is making me incredibly sad and disgusted... When my girl was in her 1 gallon, every other day she was having a 100% water change... That's just not okay what you're doing. Have you even tested your water on a regular basis to see what is going on with it? You don't "care/worry" about ammonia and pH? Why is that?
Maybe you should seriously rethink your situation, and stop your friend from buying more. If all those non HMs are gone now, leave it at the 20!! Get 5 tanks that are 20 gallon longs, divide into 4, and then your friend will be set... that's only 5 heaters the friend will have to buy, 5 filters, and 5 tanks, compared to 20-ish.. Honestly, you could probably fit in 5 fish per 20 and have 4 tanks/heaters/filters. Can you even properly put in decor, hiding spots, etc in a 1 gal jar??
The only time I will EVER find these kinds of set ups acceptable are in breeder homes that have excellent set ups (drip system water changes, fully heated rooms, carded, etc..) or people that actually maintain their bettas properly in the 1 gallons... i.e., more than one water change per week!
Sorry if I'm coming off harsh, but I honestly don't feel you understand what kind of situation your friend and you are in...