Restarting Sort Of

Let me see if I have this straight.

You want to move fish from their current tank to a new one the same size, but you also want to use the current one in the current tank soon - more fish?


As long as you have kept up with weekly 50% water changes, the water currently in the tank should be pretty much the same as tap water so there is little point moving the water over. Just move the filter and the fish to the second tank.
If you plan to use the current tank, put some of the current tank's media into the new filter, and some of the new media into the current tank's filter and run them both with the flow rate turned down if possible. But it will take a while for bacteria to colonise the new media because with the same amount of fish, there will be the same number of bacteria, and they live in the biofilm attached to the current media so there is no reason for them to move into the new media.
The other alternative would be to put the old filter in the new tank with two thirds of the media and one third new media. then put the new filter with two thirds new media and one third old media into the old tank and do a fishless cycle. With a lot of bacteria in the old media it will go quicker than usual.

The other alternative would be to put the old filter in the new tank with two thirds of the media and one third new media. then put the new filter with two thirds new media and one third old media into the old tank and do a fishless cycle. With a lot of bacteria in the old media it will go quicker than usual.

This suggestion from @Sue still seems right to me based on what you have said. However if you are talking there being some time before the current tank is restocked please note, you will need to keep the tank fed with ammonia even if not doing the cycling initially to keep the bacteria alive and healthy.