i would make sure i sealed it really well by laying a nice thick bead on all the cracks then smearing it in with my finger. then make sure u allow it to cure for i do believe its 48 hours b4 u add any water it will say on the tube. make sure u use 100% silicone also. i hope it works for ya.
Not to sound like a complete A-Hole here but.... wouldn't a replacement 20g tank cost about $20.?? How much did you spend on the silicone..$5. $8.?
I think I would invest another $15.00 - $12.00 for another 20g even if you can get a used one rather than take a chance on this one breaking and causing not only damage to your home but killing it's inhabitants. How much would all of that cost you?
Plus, think of it this way....... Would you feel completely secure on your 1-2 week vacation knowing you smeared some silicone over a cresent moon shaped crack in your aquarium?
If it wasa big tank I'd go buy a sheet of glass that fitted in the inside and glue it with a lot fo silicone, but for a 20 gl. I think it would be safer and cheaper to get a new one.
You could also go the the glass shop and ask for a glass of sheet 3" wide x lenght of tank, and glue it with a lot of silicone and then sela the sides. This would work as a a reinforcement and the tank would be atronger then new, but it's also expensive and time consuming.
My best advice is to get a new tank. Cracks only get bigger as time passes.
Well i have decided to turn it into a river style tank. You think that small crack can hold like 35% of water in it? Since its gonna be half water and alot of land?