I was thinking like a betta and some livebearers as they don't need to be in large groups, only about 3+. The reason why I said that is it has a longer foot print so there is more room and substrate for bacteria than a normal 6.5 gallon.
Yes, the volume of water in a fish tank matters, but another important thing to remember is that a tank with a bigger floor space will have more area for good bacteria to be housed. As long of course you have a substrate. It will also depend on your Betta, as don can b with tank mats othrs can't. Remember bettas are aggressive fish so tank mates should be compatible.
I saw this tank at petco yesterday, they actually have a better deal for a 10 gal in store atm. I was gonna get the 6 but I saw the 10 and couldnt resist. Got a 10 gal for $40. Came with tank, hood, fluorescent light, filter and a 50watt heater. :) it's this tank.
I have this tank. When I first got it I divided it and used it for two bettas. I would recommend getting another filter for it though. The one it comes with is really loud. Now I use it as a spawn tank. I do think it's a pricey for what it is though.
I have this tank and I love it. It narrower and longer than a ten-gallon, so my betta gets a good long swim. I was going to divide it for my two bettas, but Leonidas loves it so much, I didn't have the heart. I just put a lot of plants in there, as well as a big piece of driftwood thathe can swim under and around, and then set the filter on a very low flow. He's been very happy!