Congratulations and deciding to change your plan of action ^^
No matter what, I HIGHLY doubt you'll succeed in a small peaceful community tank. But there are some ways to be more efficient with bigger tanks. I have a large range of silly, silly obscure ideas I'd like to throw out
#1: A kiddie pool. One of the ones about 4-5 feet in diameter, and about a foot and a half high. It's inconvienent for size, but I think if you can get around the jumping, this would be an ideal betta community setting... The major issue I see are filtration. You would need to buy a pong filtration system. I would suggest leaving at least 7 inches of space at the top without water, to help prevent bettas jumping out of tanks. I hope to have this one day, indoors, until I can afford to build an indoor raised pond (this is of course after I get my own house, and then add on a large greenhouse-type room that shares the heating
#2: A high under-bed storage bin. They come in so many sizes. Really, hight of tank is VERY unimportant.... You could probably get one 8 inches high, 4 feet long, and 2 feet wide, for a GOOD price... And in reality, it is not much different then a tank these dimentions, but 4 feet high, for bettas. You can get them crystal clear, and put them on a high table or a dresser. Lids come with
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Again, filtration is the only problem. Maybe a large in-tank filter.
#3 Sucking it up and buying a really big tank. My random guess is that the bettas are going to try and claim their own territory, and each territory wou8ld be at least one square foot... So you would neet 1 square foot per fish if you don't want much fighitng. Also, you probably need a LOT of fish to spread out the aggression.... 7 or 8 at the least. You would need a large tank. (this paragraph is pure guessing, but I think it is probably fairly accurate)
#4: Switching to females
I would imagine the only way you could convince bettas to not fight in a smaller tank, is to convince them that the other bettas are NO threat to their territory... I think the only POSSIBLE way of accomplishing this is to raise the whole group together from birth, and even then it is highly unlikely.
Given, research and trial-and-error is a good way to test these things, and find loops around them. I completly support such research, because I think a community betta tank that was truly peaceful would be a sire for sore eyes
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I can see you don't want a larger tank, so you have a VERY daunting task ahead of you!
Good luck!
Katy
And again, thank you for reconsidering your community plan!