You may find it takes quite a bit of effort to get every trace of old silicone off. It can be a pain, to say the least. But if you miss any, the new stuff will not stick to it and you will have to do it over. I usually fill the tank and let the water show me any silicone I missed, it will either move in the water or look a bit whitish. I bought a box of 100 razor blades for the scraper, and while I did not need that many, it was cheaper than buying them in five or tens, and you will have to change fairly often, as they don't work very well once they get a bit dull.
Btw, the silicone on the inside of the tank, that is applied over the joins, is not absolutely needed. So long as the seal between the panes is good, the tank will hold water. Some high end tanks don't have any sealant inside them at all, and look very nice that way. But it is a bit of peace of mind, if you have concerns about leakage.
Also, when you are scraping off the old stuff, be very careful that the blade you use does not slide down into the seam between the panes of glass. That's what is actually holding the tank together and you don't want to compromise that hold. Don't cut straight down past a glass pane, only to where it meets the next pane of glass.
I find a long handled bent razor scraper works better than the cheap molded metal ones that have handles about six inches long. But an extra, very short scraper, two or 3 inch handle, can be very useful for corners or snug spots. If the tank has a plastic frame on the top, remove it. You can replace it later, and use enough silicone with it that it will never wick water over the glass if you overfill the tank sometime. Might have to use the razor to remove it, but they are not usually attached really tightly. Just a few spots of silicone is typical.
A plastic scrubbie can be helpful to remove traces of old silicone too. Be sure it's all gone before the reseal. If you use tape to get nice straight lines, and I recommend it if you have never done this before, you must remove it right away once you finish. If you allow the silicone to begin to skin over, when you pull the tape, it will take the silicone with it and you will have to do it over again. Pull off the tape the instant you finish the job. If it leaves a few smears on the glass, you can easily scrape them off later when they dry, so don't worry about them.
Once it is dry, and before you spend time fixing any smears or rough bits, refill the tank and let it sit for at least 24 hours. I prefer a few days personally. Just to be sure you got it right and the tank does not leak. Then clean up any smears and you are good to go. If it does leak, you get to do it over. Such fun
. I've done 3 tanks so far, all have turned out pretty well. I found a vacuum useful to suck out the loose bits of scraped off silicone, btw. Easier than trying to pick all those sticky bits up by hand.
