That's a nice size tank for a betta and a few other fish, though 48 litres doesn't leave you much room for many other fish. Have a look at pygmy cories (you may have to check a few shops to find them). These fish swim in mid levels more than other kinds of cory and they only grow to about an inch so they are the right size for the tank. If you did decide on the cories, you would need small smooth gravel, or even better, sand on the bottom of the tank. You might also be able to get away with a few endlers, though as bettas can have quite variable personalities, you could end up with an aggressive one which would attack them. The only downside to endlers is that for both males and females you need at least two females for every male, so the most you could have in your tank would be 2m 4f. Females are plain beigey grey and have fry every month. This shouldn't be a problem as the betta would enjoy the snack so you shouldn't be inundated with baby endlers, but there's always the chance of a lazy betta who will leave them alone, resulting in a massively overpopulated tank. Half a dozen male endlers should be OK. I have a group in my 125 litre and they mainly ignore the other fish, swimming round as a group.
It is usually advisable to get the other fish first and get the betta last. If he goes in first, he could see the newer fish as invading his territory. If they are already there when you get him, he'll regard the other fish as part of the territory.
Decor for bettas should be smooth. Those long fins are quite delicate and easily torn, so rough rocks/caves/logs etc should be avoided. Take an old pair of tights to the shop and run them over the decor that takes your fancy (or if you're a man, take your wife/girlfriend/sister as you might get some funny looks ) If the tights snag on the ornament, don't get it. Plants should be real or silk, plastic plants can have sharp edges.
Bettas like to swim through things (make sure there are no holes just the right size to get stuck in, yes they can be that stupid) and a tallish plant with flat leaves is a good idea - bettas like to rest on them as lugging those fins around gets tiring, and if there is a leaf near the surface they can reach the surface to breathe without much effort.
Just in case you don't already know about it, are you aware you need to cycle the filter, preferably before getting any fish? You can read up here http/www.fishforums.net/index.php?/topic/113861-fishless-cycling/
Edited for spelling.
It is usually advisable to get the other fish first and get the betta last. If he goes in first, he could see the newer fish as invading his territory. If they are already there when you get him, he'll regard the other fish as part of the territory.
Decor for bettas should be smooth. Those long fins are quite delicate and easily torn, so rough rocks/caves/logs etc should be avoided. Take an old pair of tights to the shop and run them over the decor that takes your fancy (or if you're a man, take your wife/girlfriend/sister as you might get some funny looks ) If the tights snag on the ornament, don't get it. Plants should be real or silk, plastic plants can have sharp edges.
Bettas like to swim through things (make sure there are no holes just the right size to get stuck in, yes they can be that stupid) and a tallish plant with flat leaves is a good idea - bettas like to rest on them as lugging those fins around gets tiring, and if there is a leaf near the surface they can reach the surface to breathe without much effort.
Just in case you don't already know about it, are you aware you need to cycle the filter, preferably before getting any fish? You can read up here http/www.fishforums.net/index.php?/topic/113861-fishless-cycling/
Edited for spelling.