Hello,
I happen to have 2x 2ft by 1ft fish tanks, I'm thinking of pulling the fronts of and sticking them together with no-toxic sealant to make a huge one? any advice?
No reason why not after talking with the people that made mine when i collected it. One proviso is most of the strength in a tank comes from the base, if you look at a tank all the glass actually sits on it to take its weight.
You will need to remove ALL the old silicone. Then get a new 2x2 base plate of float glass of at least 6mm, 10mm would be better and then build it back up. Building a jig would also be necessary to get the glass square.
Obiously aquarium silicone is a must, non toxic may not mean non toxic to fish. A lot of silicone is anti fungal or antibacterial.
Before you got to all that trouble ring Wharf Aquatics in Nottingham (providing your in the UK)for a quote

You may find its only 40-50 quid for a tank that size without any worries.