I take my tanks to the local natural gas company dock where they use compressed natural gas for their vehicles. I just gave them my adapter and they filled it up for me for free!! At there residential prices they said it would only cost a few dollars at most to fill it but since they had no way of measuring it they just let me have it. It probably would not be a go idea to tell your marina mates about this.You may want to try and fill the CNG tanks at a commercial trucking warehouse that uses CNG for their forklifts in lieu of propane. We have no vehicle fill stations in my area and a friend made that suggestion to me, they were nice enough to fill the tanks after I supplied them with the adapter for the fill. Fill them slowly, as with the previous post the tanks will warm up considerably. Just take your time and the 3100 PSI is achievable.