It seems to me that this is really a question as to how you advertise. I assume that you have the appropriate equipment (test kits, buckets, nets, python type vacuuming device, turkey baster, etc.) I've seen people put up ads at LFS. I would imagine that an announcement at your local aquarium club would work too. Check the phone book in the yellow pages to see if others advertise there.
Lastly, find someone who does this that is a good distance from your area (someone you won't be competing with for business), and ask them how they got into it.
The only people I know who do this are actual LFS employees, who do it on the side.
Keep in mind that if you clean someone's tank, and fish/coral die the next day, the owner will probably try to blame you.
