JamesM said:http://cgi.ebay.co.uk/CO2-Carbon-Dioxide-Diffuser-Dispenser-Aquarium-fish_W0QQitemZ110493386542QQcmdZViewItemQQptZUK_Pet_Supplies_Fish?hash=item19b9eb472e#ht_2120wt_1165
These work really good imo. Similar products are also available on ye olde FleaBay too