Im sure other people have written similar things but I didnt find them in a quick search so hopefully this will be useful to someone.
I am a DIY novice, my experience so far is limited to flatpack and putting up shelves. Honestly if I can do this anyone can!
I started by looking around the internet for guides and there are lots out there. I found enough contradictory advice to make my head spin. This is the guide I ended up using.
Its a really good basic guide but I did find a few problems with it.
1. The 3 hours he says it takes is a bit optimistic. Where the old sealant was badly applied it took me ages to remove it properly. I suppose you could just be less thorough when prepairing the tank but previous experience with other adhesives tells me this is a very bad idea. The better the prep work the less likely you are to have a wet floor.
2. Once most of the sealant is off you are going to have to clean the glass. I tried with a washing up sponge/scourer as suggested in the guide but the silicone sticks to the sponge and its really hard going. Instead I used a microfibre cloth. This revealed more smears of sealant so I went over those bits again with the blade, it came off easily once some pressure was applied. I repeated that until no silicone was left on the glass and the tank looked clean. Things really speed up once you get to this point.
3. I know the guide says to use masking tape to mask your silicon but mine was continuously pealing, curling up on itself and generally being temperamental. Perhaps the glass was too cold to stick right or maybe my masking tape was rubbish, but I got really annoyed. The parts that the masking tape stayed down on I left but I used sellotape for most of it. The sellotaped sections turned out much better. If I was doing it again I would definitely use sellotape for the whole thing. I used a normal clear sellotape but it would be much easier with a matt or coloured tape so you can see where it is.
4. I did this inside (its about 19 degrees in here) and my silicone was drying really really fast. Its probably easier to do this somewhere cooler but if you can't then I would suggest working quickly and smoothing each seam before moving to the next one. I was very glad that I did this bit in the morning after a good nights sleep and a cup of coffee :)
PS. This is mentioned in the guide but I just thought I would clarify as it took me a while to get the technique down:
When making the first cut into the old silicone its tempting to try and push the blade in flat against the glass. This makes it really hard to get a good grip and you will get a lot of resistance. I found that it was much easier to have the blade at approx 45 degree angle so only the sharp edge is against the glass. Then you can start teasing it under the old silicone with a chopping motion. It should come off fairly cleanly.
Thanks Onyxia, that's very handy. My bro-in-law did a really good job on one of my big tanks for me, but he's now had a little quarantine tank sitting in his garage for months, maybe I should get it back and have a go myself, you've given me confidence to try!