1. Do all the seams on the inside. If some of the seams are getting old & brittle, the others will be in the same shape. May as well do all the seams now to save you from having to tear the tank down & redoing another seam a year or 2 down the road.
2. Remove all the inside seams, but no need to take the tank apart unless a pane of glass is loose. Make sure you cut the seam off rather than pull so you don't damage structural seams. I find a sharp skew chisel much easier to work with than razor blades (no skinned knuckles) ... that is if you have one!
3. After cleaning all the area - remove all silicone, clean with vinegar for water deposits, then 100% iso-propyl alcohol, let it dry completely - tape the seams with masking tape.
4. After laying dowm the silicone (yes, GE 100% clear silicone door & Window without mildewcide is what I used) , smooth with a (smooth latex) gloved finger. <Seaming tool will work too, but fingers are cheap! Remove the masking tapes while the silicone is still wet (you have 10 minutes or so) and you will have a perfectly straight & tapered seam.
5. Wait 7 days for the silicone to cure, then leak test.
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.