I had a single female betta living in a 45 gallon tank for a couple weeks before her tank mates arrived. I would find her swimming at all levels of the tank and even with a strong filter current she never had any issues. The tank was around 18 inches from top to bottom.
I think HM males can go into larger tanks (over 10 gallons) but they do need to build up proper muscle tone first so that their excess finnage isn't constantly dragging them down. I recently purchased a young OHM male, who has been hanging in a tank with a stronger than normal filter current. When I first got him he kept getting sucked back up against the side of his net, but now I find he is able to get around a lot more easily and spends a lot of his time swimming rather than resting. His finnage is quite large in proportion to his body but I have not seen him take a single nip out of it.
While this is obviously not the case for everyone, I have never had a single betta of mine dislike being in a larger tank. This includes everything from VTs to HMs. I have always thought bettas to be quite an active fish once they have enough space to swim about in. Even my imports have been happy when provided with larger accommodations. In fact, I have had more tail biting incidents from my males when kept in smaller tanks than I have with those who have been 'upsized'.
I had Vilkas in a 10 alone, and he ate away his ENTIRE cadual fin, but as soon as I divided his tank, he stopped. But you can also say it's because he has a new friend, Brynjolf the little warrior, but I'd say he likes not tugging his fins around as much.
Again, he'd have been better if he was a tag bigger, he's small for a boy.
I keep my betta in a 31,5g tank (120l). He's healthy and happy, he's eating well, making nests and exploring the whole tank. I don't think a tank can bee too big, if only it's decorated right. I use a lot of plants and floating plants and moss. I also have coconut caves and big roots.
In Norway experienced people recomend atleast 15,5g (60l) for female bettas, and community tanks. I keep one male CT in a 5g tank, and would love to give him more room. I think that it's important to not have any other fish with the betta in such a small tank, exept maybe snails and shrimps. But, I don't think 5g is too small for a betta. Bettas has different personalities, and I think some of them feel safer in a smaller tank. I would not use anything smaler than a 4g for my bettas. For me the ideal size would be 15g.
I have also kept several male bettas in a 53g. They were not stressed out at all, and I have never had problems with any disease or tail biting.
My favorite betta, Unicorn has a 5 gallon all to himself. He sometimes explore around for an hour then he'd get tired and go to sleep under the roots of java fern. ( I observed him for a while) Also I plan to add big ghost shrimps and hoping that Unicorn wouldn't eat them lol.
The other four betta's in 4 ways divided 10 gallon tank... they seem okay but I plan to set up other 10 gallon just for two bettas.
I think bettas do best in 5 gallon or bigger. If you have a big tank to give a betta the good life, go for it.
I know when people say "too big" it's usually those who think they are happy in half gallon cubes, but I actually want to know. If proper the amount of cover/hideys are used, can a tank be too big (uncomfortable/stressful/etc.) for one betta?
My betta Beau was completely stressed in his 5.5 gallon...after two weeks on clamped fins, no eating, and erratic swimming, I put him in a 1g bowl. In two days he started eating, swimming normally, and colored up. I think all bettas are so different in personality etc. It can happen...I have proof.
Mine range from just over 3 gallon to 7 gallon tanks. All have a single male in although I do have a baby bristelnose plec in the 7 gallon who I am thinking of rehoming to a larger tank and 4 vitnamese cardinals. They all have a cave and lots of live plants and moss balls and I have just invested in some betta hammocks and betta logs which have arrived today and will be put in tonight. Each one seems happy in their environment
Buddha is in a 2 gallon at the moment, and he actually seems to really enjoy it. It's making me wonder if a 5.5 gallon will be enjoyable for him. He only swims in around 1 gallon of the tank. He swims at the top of the tank and only goes to the bottom to sleep, or to hide in his log.
But he's going to have to like the 5.5 gallon because i don't think I can have more than one tank...