If this is your only argument as to why people should buy from your local dive center, you need to re-evaluate your business model.
If the only value you bring to the diving industry is that you can fill tanks, you need to realize that compressors are relatively cheap compared to the overhead of opening and operating a dive center.
A group of independent instructors could probably all throw in and buy a compressor.
Airgas has 1,100 locations and lists “medical air” on their web site, so I would imagine that would be safe in a SCUBA tank. My local branch says they can fill tanks, but I haven’t done any independent testing of their air.
If you can’t claim better service, better prices, great value added services, contributing to the community or anything beyond air fills, are you running your business successfully?