I will start off with the same disclaimer as Marcarl, I will be very interested in the opinion of others, I'm not very savvy when it come to rusty tanks and what to look for,,, but,,,, I would be sure it is disconnected from the system making sure the petcock is open and wipe it out and then blow it out with compressed air, flush it out with water until it runs clear and watch for any sign of the tank leaking while it is full of water. Then wipe it out and use air to help dry it best you can, connect the line with the old filter in it up fill it with gas and double dose of sea foam and drain that tank through the filter into something you can see the gas in to check it's condition then replace the filter connect everything up and refill the tank after running out that first tank change the filter and monitor the new filter and change as needed. Just my two cents