I have done tanks for myself and also others in the same situation.
DON'T get a shop or other, where you are paying, to do the re-siliconing,
unless you strip it down yourself.
The cost of removing the glue would equate close to a new tank cost.
This job, to do properly, is slow, and does have a few risks.
I use a "bik" shaver that has been pulled to pieces.
The blades are so small, they fit in all but the best joints.
I break the blade-holder sides from off the handle of the shaver, and use the handle and the stub,
to push the blade thru the silicon, if you don't do this, expect cut fingers, or wrecked finger-nails.
The job of disassembling is not so time-cinsuming, it's the removal of the old silicon from all joints.
The final part of the way I do it is to roll off the fine amount left with the palm of my hand.
If I'm not happy, I use my spit-stone that I aris the edges with to get to the glass.
I'll also use it to buff the glass for better glue bonding (fine scratching).
Now you have it this far, the easy part it there, It should take an expert less than quarter of an hour to do the gluing.
Now about the octo in the tank, I am sure I have read that if they "ink". they can make the tank so toxic, that they can kill them self.
Don't know if this is true, but it's worth looking into.
I went to Portobella Aquarium, and they told the story of a self-help-octo that climbed from it's tank at night
and went food hunting in another tank, and to return home before the staff arrived to work.
It was only discovered after cameras were set up.
To octo safe the tank, I'd make tops that would slide into a gap created by a couple of pieces of glass,
glued on each end brace, to form a Z.
Get what I mean??
Alan 104