The bb in your old filter should be fine to add to 180 along with the entire stock. You should also transfer the substrate and all the decor rom the 75 as well as they have bb on them also. Here's what I would do: Fill 180 to about 70% with new water; get the new water up to temp; transfer enough of the 75's water to top off the 180 and transfer all the decor and substrate because they have bb on them as well. The old water is not so much for introducing bb, but rather to have a little bit of ammonia to feed the existing bb from the filter, substrate and decor. Continue doing same water change routine that's been working for you, but keep a sharp eye on the water parameters. After a few weeks, your new filter medias should have established a bb colony, and at this point you can start removing any decor or the substrate you don't want in the 180.