Just to agree with everything FHM said, Yes, unfortunately the MM can harbor pathogens, so its better if the tank you receive from has been disease free for a longer time. When you find a candidate to supply you with some MM, its a good time to share the things you've learned about the nitrogen cycle, fishless cycling, MM and best practices of gravel-clean-water-changes each week and monthly filter cleans as this is a good way to help spread these good practices.
Work with the person to identify which media in their filter would be the best for the transfer. There is a "pecking order," with ceramic biomedia (ceramic gravel, ceramic rings etc.) and sponges being the biomedia that will have the highest levels of biofilms and bacteria. Carbon and polyfloss, the other two media you might commonly run in to would have somewhat less (but of course anything is better than no MM at all!) Of course, don't take Zeolite if you are able to determine that the person has that as it is a chemical ammonia adsorbent and could deprive your bacteria of food.
Once you've identified the best media to transfer, obtain the new replacement media you will give the person in exchange. In all cases no more than 1/3 of the biomedia should be removed from the donating filter. That will allow it to continue operation without the donors tank seeing a mini-cycle. Obviously, you are going to often have to be fairly "creative" about fitting media meant for a different filter into your filter. Do not overstuff such that flow is blocked off but in general just try to use common sense. Ideally the MM should be in the water flow direction just prior to the main new biomedia of your filter. If you are adding cut-up pieces of sponge from the donating filter and your filter has a major sponge then try to place it just prior in the water path. Or cut your sponge open in some way and insert the MM pieces into it. If its loose media, such as ceramic gravel then its great to mix it up with your loose media tray. In general, direct contact is superior to all other juxtapositions.
Don't go and get MM until your fishless cycle has been going for a bit and you are comfortable with the number of ml's of your particular ammonia solution that allows you to acheive 5ppm or close to it, assuming you are in the first phase of fishless cycling prior to the nitrite spike. You just may have better results if the ammonia is definately there and getting all through the filter when the transported biofilms first get there. MM transplants are not always a sure thing, its fairly unpredictable whether they "take" and I don't think we know all the conditions which optimize this, but overall its definately always worth a try or more than one try even.
Good Luck!
~~waterdrop~~
