Thanks for the help The tank is only really really tiny. Only big enough for 1 fish IMO. My Sister just bought the kids starter kit and we all know how unsuitable starter kits usually are for ANY animal. The tank is only 13 litres.
I put the second filter in as hopefully at some point I will upgrade to a bigger tank then I can have the second tank cycling with a mature filter whilst still keeping the fish in the titchy tank until it's done.

Anyone know of a place to get cheap fish tanks? Maybe I'll try freecycle...