I inch per gallon of water which is.... 3.78 L. So if your fish is almost 2 inches long.. 4 cm? Then you are at the limit, it will not grow any more if they only grow to their environment.. if it grows you need a bigger tank.

No, it's not big enough. He may be a small fish that is able to breath air, but keeping the water quality stable in a tank that size will be nearly impossible. For a betta, I would recommend the following: A 5 (UK) gallon tank (about 6 US gallons) A 25 watt heater A sponge air powered filter A few plants and some gravel. He will not grow to the size of the tank, nor will any fish. This is a myth. If fish grew to the size of the tank, I would have myself a pet great white shark by now lol. Make sure to cycle any new tank *before* you put the fish in, using fish to cycle is cruel and unnecessary. Small tanks will cause problems firstly with water quality (ammonia burn, nitrite poisoning) then will come the secondary diseases (finrot, ich, bacterial infections, protozoan parasites) so it is worth starting out with the right equipment to prevent problems in the long run. A lot of pet stores give spectacularly bad advice about bettas, so it's best to do your own research. Just because they can survive in small containers, doesn't mean they should be kept in them. In the wild, bettas can survive in puddles, but they actually live in streams and rice paddies, which are massive if you see a picture of one. Here is some basic betta care info- http://www.thetropicaltank.co.uk/Fishindx/betta.htm A couple of my friends breed these and they swear by 5 gallon tanks!

It depeneds on the size of your fish. If it gets any bigger than six inches long, you need to get a bigger tank.

i think that is enough i have a betta and he is in a 6 or 7 L tank but they can jump out of the tank if the water is to high so leave about the lengh of your fish at the top of the tank hope this helps