I used the GE II for doors & windows. I have thrown away the 2 tubes I used and that was all I bought. I didn't realize they had an expiration date. What happens to it after that date? I have another tube that is made by DAP that has a D.O.M. date that I am assuming means Date of Manufacture. How long is it good for?
we usually use the GE Silicone II for doors and windows, the DAP, I'm not familiar with.
Like Dane said if it is a bad tube, it won't dry.
It seems like when they start approching the 1 year date they seem more likely to go bad. Check the date on the tube of DAP and if it is close to a year, you might take it back and exchange it for a "fresher"tube.
Depending on how much of it you used, the temperature where you are, and how you applied it, it is entirely possible that the silicone isn't cured. Touching may tell you that it is cured at the surface, but it won't tell you anything about the silicone that is behind the bark or several inches deep in a crack.
Room temperature vulcanization silicone (RTV) normally gasses off acetic acid as it cures. At room temperature, acetic acid is a liquid so some of it may return to a liquid while some may evaporate almost immediately. Acetic acid evaporates more quickly than water, but not as quickly as other things like rubbing alchohol. If the temperatures are low (like you have the aquarium curing in a cold garage) or the silicone is trapped behind the cork bark, it may be curing little by little off-gassing a little acetic acid continously.
Similarly, since an aquarium is a somewhat closed system, the evaporating acetic acid could recondense and not leave the aquarium very quickly. Add to this the fact that the cork might be retaining some of the acid, and it is possible that your silicone is working as advertised, but curing slowly. I would make sure that the tank is in an area that is somewhat warm, wipe down any moisture you find, and add a fan or something to circulate as much air through the enclosure as possible.