That is a bit small but like DQ said, it's something that you'll have to do very frequent water changes with to keep your fish healthy. It's not a size that you can put a heater into so is your fish in a warm part of the house?
Have you thought about getting a critter keeper if you don't have the money for a tank kit? They're very inexpensive and you can put a 25w heater in a 3 gallon one with no problems.
I'm just trying to make some suggestions here for you.
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I have always kept mine in a 10 gal all to himself, but a step-daughter has one with her while away in College in a 2.5 which is the max (3gal) allowed. I find it tight but livable and 7 months she's had him now and still thriving, with filter and heater.
But having said that I have also kept a single Betta in my old 45 Gallon. Yes and HE LOVED IT. I think he thought he lived in the ocean, and he owned that ocean.
My point is that we should try to provide the biggest home we can for them, they are in our care, it's our duty to do the best we can, in spite of what your LFS says is ok... after all they just want to sell stuff and make money after all. Some people just don't say it very nicely I'm afraid. And I think when the bag of water your fish came home from the store in is BIGGER than the tank the fish is going into... what does that tell you? Would that bag really have been big enough to live in? Seems funny doesn't it when you think about it that way.
And a little more space...means a little more decorating room too!! You really can have a lot of fun with it!!
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