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I guess its safe to say that after my couple hours of here and there research backing up with real experience... that bettas, some tetras and such that grow up to 2 inches are perfect in a 5 gal. tank! There isnt such things as "sufficating" fish... its just overcrowding that stress's them and such! My freind (true story) kept 4 female bettas in a 5.5 gal. for a year and a half, and they lived happily from the day he got them to the day he sold them becuase he went to an out of town college!
I personally own a 5 gallon with 3 female betass, (thinking of getting one more) and 4 small 2 gallon cubes on my shelf containin a male betta each. I say that if you were to get a more crowded 5 gall. (3-5+ fish) the more water changes needed. I think other from stressing them out, if there happy in a large number in a small tank, then the water quality would be the issue, not the "sufficating" and such that people learn from the jacka**'s from the store who want you to buy a huge tank for profit, and so you dont want a refund!
Word of advice to all who read this... be carefull what you read online, and what advice you get from the petstore owner/workers. Especially the petstore ppl, becuase many of them think they know what there talking about, or dont have a clue and sort of wing it and try to sound like they have a clue. I had one guy say to me that I couldnt keep 5 female bettas in a 10 gallon, and they would die in a week tops. I wanted to buy him a book to read on fish lol. Remember all books are like the internet, you cant trust them all either. Especially books published 4-5+ years ago.
If you had time to read all that, thx and hope this clarified some of the true answers and tips ppl have been giving!