No it isn't suitable. It's do-able but will be a lot more work for you and more of a health risk for your fish should you be unable to do a water change for some reason. The smaller the tank, the less room for errors and the more unforgiving the conditions. A betta can certainly survive in a 2.5 gallon, they can survive in far less.
But this is no reason to keep them that way.
No one with an average home or small apartment is so stuck for space that they can't find room for a 5 gal tank.
All tanks need to be cycled regardless of size. Though it's actually your filter which cycles, not your tank.
Instead of wasting money on a 2.5 gallon which is far too small, why not go for a
second hand 5 gallon. You're not obligated to buy a brand spanking new tank from a shop.
Have look on Ebay, Classified/free ads, Craigslist, car boot/garage sales, ask round the neighbors and see if anyone has a tank in the garage/loft they no longer want. All these sources can get you a cheap or possibly even free tank. 5 gals is a common small size tank that's just right for a betta. Gives a good minimum swimming space and is far
far safer than a 2.5 gallon.
If you have room for a bigger tank than 5 gals and see one cheap, then get it!!! Contrary to popular belief bettas do NOT like small spaces. That's a common myth devised to make peple buy tiny gimmick tanks. What bettas actually dislike is OPEN spaces. And that's related to how you set up your tank. (see next paragraph for why )
Plants are prettty much a nessescity.
Wild bettas come from waters which have a lot of plant matter , dead leaves, fallen twigs ect ect. They like a lot of places to hide. Many would give the argument that tank bred bettas " are not wild" ( no, really? ) true of course but they retain all the instincts of their wild fellows . But it also means that they tend to dislike open bare spaces, tanks with very little planting and so on.
There are plenty of easy plants that require nothing more than a normal tank setup and a bog standard aquarium light. No complicated ferts, no CO2 ect. Cabomba, Elodia, Echinodrus, Aponogentons, Java fern, java moss, riccia (floats) and moss balls. These all grow well in basic tank conditions.
One tip I would give is to have a thin layer of bog standard , cheap
pond soil under your main substrate. It does seem to work better topped with sand I've found. The pond soil will provide some basic nutrients to get the plants started and over time the waste and mulm from the fish will help greatly. I have around 1 cm of soil capped with around 2 and a half inches of sand. My plants all grow well in that
Also, ifyou don't feel brave enough to try real plants, get a load of silk ones. Really fill the tank don't just get a couple. Again. ebay is another good place. You can get packs of them on there which works out cheaper than buying them individually.
