I would probably try a different approach with that new tank Stuuk. Since you intend to retain the old tank and filter for some fish, try just using the new tank, with its filter running and water dechlorinated, as the cleaning bucket for your cycled filter. Then you can add a few doses of ammonia to "fishless" cycle the new filter using that huge jump start that a clone gives on a new filter. My own experience doing things that way is that the new filter is cycled in just a very few days. I do not even worry about getting a tank ready for new fish these days. Instead I wash an old established filter in a new tank and add fish as soon as the water starts to look clear. The tank is a bit messy looking at first but the fish really don't care and the water chemistry stays good from the start. With 25 tanks running, there is always one filter that badly needs cleaning, that is the one I use. Tonight I actually cleaned a filter without cloning a new filter in the process, but next week, when I come home from the ALA convention, I will probably need to clone several filters. So be it. I have plenty of semi-dirty filters to choose from. I even have tanks with more than one filter running on them which can just be moved to a new tank.