Since I had been wanting to replace this old galvanized tank with a newer bladder-style version for a long time, I decided to replace everything, not just the fittings. The problem though, (and the reason I hadn't replaced the tank before now,) was that despite wrapping the tank base when we poured the slab, the concrete still "glued" the old tank in place.

I tried an impact chisel first followed by yanking it out with the golf cart but but both attempts were futile. The only remaining option was to cut the tank off near the bottom and then dig out the base.