Try methylene chloride. Available from paint shops and electronic outlets, used to dissolve epoxies and just about anything else that polymerizes. But it sounds like you may have a rubber based sealer, methylene chloride will soften it but won't do a very good job of dissolving it.
BE CAREFUL WITH METHYLENE CHLORIDE!!!! It's very toxic, don't get it on your skin or inhale it. Safe enough if you work outside and wear PVC gloves, no need to be OCD but do be very careful. Pour some in the tank, let it sit for a few days, then empty it out and toss in a handful of drywall screws and spend a few hours just turning and shaking the tank. The screws will remove any loose sealer (and get rid of the rust that was the reason the DPO put the sealer in in the first place).
You can expedite the shaking with drywall screws if you have access to a concrete mixer, just pack the tank well with lots of old blankets etc and just let it turn for as many hours as necessary. You may need to repeat the process a few times, and you may even end up with some undissolved sealer in the tank. The tank may have got so rusty it may have pinhole leaks, which you can try to braze closed (careful of explosions!) or you can use a better quality sealer. I very highly recommend Caswell's tank sealer, just Google it, epoxy based, easy to use, works flawlessly. I do recommend thinning it about 10% with acetone or xylene to make it spread easier, unless you're applying it over 30C.
Ken
