If you want to save silicone, you have to totally reseal the silicone tube (or any other caulk - except for polyurethane which must be used within a couple hours of opening). The cap supplied in the tubes are totally useless.
I found a good way to keep my caulks "fresh" is to use a lag screw. Depending on the size of opening I cut in the tube, i find a 1" or so lag screw that will fit tightly in the tip. You have to find one where the thread will cut into the plastic to get a good seal. <I usually have assorted sizes from 1/4" to 1/2" left over from various jobs. I find one to fit.> I screw the lag screw into the nozzle, making myself a threaded plug. I then use that to reseal the tube for each use. <Make sure you don't cross thread ... if the thread got broken, you messed up the seal.> I have kept caulk for a year or more using this system. <Note that silicone has a shelf life of only a couple years even if unopened ... so you can't keep it for ever.>
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.