OK.Wildcard:Guys, try to stay with me here, An air fill is if you have your own tank or return and reuse a rental tank, a tank rental is just that and they come with air. MDS has a tank rental card as well as air fill cards available. The tank rental card is a pretty good deal if you use all of them.
Let's try this.
If you have your own tank, MDDC will fill it for $1 less than normal. Meaning a 10-fill card FOR YOUR OWN TANK is $30.
Now, if you take one of their tanks for $4 (assuming the 10-fill rate), return it in 3 hours and ask to take another, you'll be charged $4.
As you can see, the tank is just a vessel for the air -- much like the take-out box is at the Chinese food restaurant.