I personally wouldn't go with less than 5 or 10 gallons. Just as I have an oscar in a 55 gallon, I would never go with or suggest less than a 75 gallon for a single oscar in the future.
My male betta has had the 29 gallon to himself now for a while, and he is very active and happy. He swims around, flares all the time. It just seems so much better to let them have room.
More water gives you more breathing room for everything. If you change water every 4 days in a 2 gallon, then you could change water every 7 days in a 10 gallon and have and keep the levels lower and more consistent. I find it is better for the fish if the range things like nitrate go through is smaller. If every week it goes from 5-10-5 in a 10 gallon, or every four days it goes from 5-30-5 ... then I know which one I think is better.
So, even though small tanks can be done ... it takes a lot more effort and attention to keep the fish healthy and happy. The closer you are to the minimum gallons and maximum fish, the more experience you should have taking care of them.
I say, if you have had bettas for a few years, or have experience with other, higher tank loading fish, then yes, you may be able to fully care for one in a 1.5 ... it may even be happy. But, there is no way I think someone with little to no experience should even remotely attempt it. They won't see the signs of a problem and know what it can be and take immediate action. That is what small tanks require, immediate reaction to anything.
An oscar in a 55 gallon will require several water changes a week. If you put this off, it can get ill. If something happens to your bacteria and it crashes, the ammonia will spike quite high very fast.
Well, bettas produce ammonia as well. They produce the same amount in a 1 gallon as they do in a 10 or 30 gallon ... but, it gets diluted by that much as well. It isn't just ammonia though, as you well know.
So, yes, you may be able to care for many 1.5 gallons tanks at once and have reasonably active fish. Just as you can see some of them flare and active in the cups at the store.
But I am talking quality of life of the betta and the ability of the betta owner.
Because some will say, my betta lived in one of the vases, planters, etc. But surviving would be more accurate. I can survive in solitary confinement in prison, pace around exercise ... even be a bit happy, but not enjoy life to its fullest.
There is no award for having more fish. For keeping 20 bettas over 10. I know we all -want- more, or see one that is beautiful while out. But, how about giving the betta the best life you can, instead of being -selfish- and getting another because it looks nice. Quality over population.
Just my 2 pence. If you have the experience and ability, fine. I don't think one should promote it ... since a newbie is not going to realize how much work it is, then, come and ask why the fins are clamped, why did mine die so fast, what is the fuzzy stuff, and so on. There is a bigger picture.
