Sorry to necro an old thread, but another thing that works to loosen up the tar is to put it in the oven if it's small enough to fit. I've did it with a 10g and a 2.5g Metaframe and it works great.
Glad this bumped up to the top. I have 3 of these tanks in various states of disrepair that I've been meaning to work on for years. Now I know how to refurbish them....
ll you be reattaching the slate bottom and adding a glass pane on top to keep it's seal, but also it's appeal? I have a 75 metaframe, the sand/dirt keeps it from leaking, though. If anything, I'll retar it before tossing the slate!