You can use silicone to seal the glass to glass seams. Silicone won't seal well to a slate bottom. Some people put a piece of glass over the slate and seal it to the other glass panels. You can't get the asphaltum any more. Some folks have used a heatgun to soften it up and get it to seal that way. Personally I filled mine up with muddy water and let it leak. After about 2 weeks the dirt sealed it up. I get a few water drops every month or so. I think it's the tank. Could be my 3 year old as well.
By tar tank I am assuming you mean one of the old Metaframes.