Slate-bottom 30 gallon

[Sploke]

I cleaned the old black sealant out of mine with a razor blade and thoroughly cleaned the slate and glass with vinegar first, then alcohol. Similar to what Sploke mentioned. The silicone sealed to the slate without any issues in my case. If it continues to leak that would be the way to go IMO.

x2, this is my method as well.

 

[Notophthalmus]

Before you scrape the old sealant out, try heating the sealant with a small torch or long-stemmed lighter, just enough that the sealant softens and the cracks seal up. Sometimes this works; if it doesn't, you can still scrape the sealant out later.