Inside the tank I lined the walls with Polyiso foam insulation board. I had extra board left over from the house build in the crawlspace. Actually, just enough insulation to finish the tank, which was lucky. There are two layers, each two inches thick, for a total R-26 on the walls. There is no additional insulation on the floor, just the existing crawlspace insulation of R-26.
I lined the tank with EPDM rubber, folding the corners, and stapling the top edge to the wood frame. I found this cost effective, purchasing from Amazon.com at $110 for a piece that was 10 feet by 15 feet, which gave me enough for the tank liner and the lid. Before bringing the EPDM down into the crawlspace I laid it out on the floor of my garage. I cut it to size, and snapped chalk lines where each fold would go. These layout lines helped me a lot when I was in the tank smoothing it out. There is a video of the insulation and liner install here: https://youtu.be/bI0T7ZegAPU
In the end the tank’s inside volume is about 250 gallons. However, with the air space, there might be 230 gallons of water.
Chalk lines snapped out on EPDM prior to installation.