I believe 10 pond is the smallest places will rent out. You will have to pay for or leave a security deposit on the tank which is the most expensive part and as you need refills you just pay for Co2, which is just a exchange of tanks. You should look up place that sell Fire extinguishers as they also fill them and most will fill tanks for vending companies. You can also try welding companies as they may know theses places and tell you where to find them. Yes food Grade, but I believe it is just standard to use it for everything, but do ask. How long depends on how large your unit is and how efficiently it use the Co2 and the biggest thing that will decide on how much is used is demand from your Tank. The more SPS the more it will use to keep the Cal. levels where you want. Make sure you have a regulator with PH probe and remember first you want to get your numbers where you want them or very close and then engage the unit to hold them there, if can be quite the task if you start to use it to bring numbers up, as you can get into a seesaw game that just goes on and on and on. I have a 10 pond unit that i would say I have used 75% of the time in 4 Years and I still have some in the Tank, it is low now but well worth the 20 or 25 bucks.
Good luck and I hope this is some help.
