11-23-2009, 08:14 PM
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I would just exchange the filter temporarily; the old tank will be fine with the new filter as there is bacteria colonizing all surfaces in the tank and they will multiply to handle the loss of the bacteria in the old filter removed. In the new tank the old filter will bring bacteria with it. But this has to be done when the fish are added because the bacteria need ammonia and nitrite and it comes from fish.
Adding pure ammonia is OK if you know how to do it. I would go with the above in your circumstances. Then, after several weeks, exchange the filters back. Unless of course they each fit either tank.