Who owns it? I live on an acreage and the old owner is trying to take an 1000 gal LP take 6 years later?

The Original owner of the acreage sold it back in 1995 to a young couple. I then bought the acreage from them in 2003. The original owner was renting out the grain bin he once owned from them as well as myself once I purchased the acreage from them. Now he wants to take the 1000 gal propane tank that is hooked up to the grain bin claiming it is his! When I purchased this place it was appraised with that tank along with a small one for the house. He has been filling it every year so I know the gas in it is his but I think the tank belongs to me now. I know he did buy it but that was over 30 years ago, and that's where is has sat ever sense. Does he still have rights to it?