The answer depends on wether you spreadbet or trade lots in the real forex markets.
In spreadbetting there is a small tax levy that you pay as part of your spread you don't even notice it ,any winnings come free of tax . Tading lots (or micro lots) will attract tax
but this will depend on your own circumstances .There is an argument put forward by
Black Swan (look back in this thread) that says if you are only making small gains you may well be better off trading micro lots in the real markets until such time as you reach a certain tax threshold (I don't know what that is). Personally I prefer to spreadbet
and not worry about it ,even though the spreads are probably costing more. I ought to add that this is an answer applicable under UK tax laws . The situation differs in other
parts of the world .