7 small fry should not affect your main tank at all unless you are overfeeding them a LOT.
You could keep the sponge filter running in your main for a couple of weeks (the fry will be fine in the net). Then transfer both over to the new tank. (this would be my prefered method). This is also why I always keep an extra sponge filter running in my main tank, might be a little unsightly but its there for those unexpected fry.
If you really want to get them out:
I don't know what filter you are running on your main tank but I would take out what ever media it has and slosh it around in a bucket of water from your main tank and then put it back in your filter . Then run your sponge filter in that "dirty" water for an hour or two. It should have plenty of beneficial bacteria for 7 small fry.
Combined with 20% pwc
every second day you should have no issue. Make sure to use a quality water conditioner such as Prime.