You seem to have done exactly what i was advised to do by my local 'chain' store last year... ignore them, never go in there again and listen to the suggestions from people on here.
You now have to do ALOT of water changes to keep your fish alive. At one point last year, i was doing 2 lots of 50% water changes just to keep my Mollies alive. It didn't work and they died a week or two later.
I also had that exact same filter you have. I found it noisy and useless. as soon as you can i suggest you upgrade, i decided on an 'Eheim Aquaball' and my small 30L tank has never looked so good! But then again, i cut my losses and chose on a complete 'fishless cycle'.
Regards,
Terry.
That filter is quite loud, hums constantly, and this tank is in our bedroom.. I also found the sucker pads come off the glass sometimes, is there a better way to stick them on so they wont come off?

/www.charterhouse-aquatics.co.uk/catalog/eheim-aquaball-internal-filter-p-1108.html