oh 5L isnt the limit it was just an idea what type of smaller colourful fish could i keep with a male betta in a larger tank?
If you can get a larger tank then by all means get it! In a 10 or 15 gallon, you could keep a group of Male endlers with a betta. They are very small bright little fish , just under an inch long ( look in the species index on this site,in the livebearers bit ) and a group of 10 will do fine in tank that size. Make sure you only get Males if you choose these though , they are livebearers like guppies and the last thing you need is constant breeding.
The Khulie Loaches I mentioned are great fish, quite pretty, but they do prefer to come out at night . Although if the tank is heavily planted, you may see them come out in te darker areas of the tank.
The Harlequin Rasboros that have been mentioend would be suitable also . They like to be in groups, and around 8 would be fine.
If you want something really stunning , then Galaxy Rasboros will look great. They are small fish like enlders, and very pretty indeed They aren't all that common though so you may have trouble getting them.
It's best not to have too many species in one tank with your betta. You don't want to stress him out with loads and loads of little fish darting about. Try to stick to one or two other species in the tank and keep it at that.
If the tank is 10-15 gals, you will need a
lot of plants and resting spots. With their long fins bettas aren't the strongest swimmers and will need to rest a lot.
Plenty of tall soft plants or a tall smooth ornament or wood is needed.
If you want tankmates with the betta, ALWAYS add the tankamtes first if possible. If you add them after the betta, he may pick on them as he will see them as invading HIS tank. If they are there before him, he will be less likely to bother them as they will be seen as part of the scenery.