I bought this similar water filter and it arrived today...http://cgi.ebay.co.uk/NEW-600L-H-INTERNAL-FILTER-FOR-FISH-TANK_W0QQitemZ160280661541QQcmdZViewItem?hash=item160280661541&_trkparms=72%3A12|39%3A1|66%3A2|65%3A12|240%3A1318&_trksid=p3286.c0.m14....but its 2 times bigger than my mini filter that came with my fish tank( im planning to buy a bigger tank for 4 fishes), My question is, is carbon good for fishes? because it has a 2 stage filter and darker water seems to come out when i turned it on in a seperate bucket with old fish tank water.