At 240 litres there is no limit on the nice small fish you could keep. Basically anything under about 3 inches that get along with each other would work. With livebearers you could have 6 of a type, 2 males and 4 females. With the cory catfish you could get 6 of the same kind and not worry about them being the pygmy size because you have room for them. If you stick with fish that are less than 2 inches long in the body you could get enough fish to add up to about 60 or 70 inches of total length not counting their tails. Of course you would want to add the fish gradually, just a few at a time, to give your tank time to adjust to the new fish before adding any more. I agree with rabbut's suggestions as far as what the tougher fish are except that I would include guppies and rabbut does not. I have not gotten any for a few years so he may be more up to date on guppies.