Restoring metal frame aquarium

In home depot or the like in the caulking aisle there should be a product for softening and removing silicone RTV. I believe its an acid based gel. Anyway, scrape most of it off and then apply with a brush and you can scrub the softened RTV off.

Why not use a clear RTV? Also, didn't they use tar originally to seal? My guess is that the silicone RTV isn't adhering to surfaces that the tar was used on. My suggestions are to add a new glass bottom on top of the slate (put a layer of foam or other support to fill the gaps between the glass and slate so they can share load). Alternatively you can chemically treat the slate to remove any hydrocarbons left over... maybe a chlorinated hydrocarbon like brake cleaner would work. You could sand the slate to expose fresh slate, and then silicone to that. Regardless of what you do I'd get some epoxy (like fiberglass resin) and paint the slate with that. That will both seal the slate and provide a surface for the silicone to adhere to.

Thanks for the suggestion of silicone remover. I didn't know there was one. I use the black sealant because I am trying to match the original (black tar as your said) and to hide the imperfections on the metal.

As to not adhering (sp?), the old sure seems to be stuck well.

Do you know where its leaking from? The slate or between the glass sides?

I believe it is leaking along the sides. At least, the water was at one corner on the bottom. The slate itself appears whole.

Well, if the joints are behind metal why not use clear silicone so you can visually inspect them?