Bad tank size for 1 molly?

No fish should be kept in a 2.5 gallon tank. Even a single betta should be kept in at least a 5 gallon.
Mollys should be in at 20 gallon tank or more. Sailfin mollys should be in at least a 30 gallon tank. Mollys are a shoaling fish and should be kept in groups of 4. Perferably 1 male and 3 females.

havens_haven said:

I have a 2.5 gallon tank ready and cycled for my male molly to come home from school. I’m not sure if this tank is large enough. Should I get a larger one? and if so, what size?

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. If you’re married to a small tank, get a Betta. If you want to keep mollies, you can find a 20 or similar mid sized tank with all the trimmings for pretty good prices, usually much less than a fancy nano tank. If you have a cycled tank, use your sponge to cycle the new tank. It will happen much faster. Then get him a couple of buddies. And beware…that’s how I got started when my kids were little and now I’m an empty nester and still addicted to fishkeeping all these years.

You can use a 2.5 gallon as a shrimp tank. I'm going to make one of those for myself one of these days.
Here's something about fishkeeping that's a little counterintuitive. Larger tanks are easier to maintain than small tanks. The water parameters are more stable. With something as small as a 2.5 gallon, not only is that not enough physical space for the fish, you don't have any room for error when it comes to tank maintenance. When things go bad in a small tank, they go bad fast.

The ten can work for mollies, at least for lone ones or groups of females, but 2.5 is waaayy to small for any fish. I’ve seen it firsthand, I am sorry to say.
And bigger is always better so long as you have the money and space, 20 or larger is ideal. Good luck✌