What gregzbobo said is true, silicone will not work for this application. Silicone does not bond at all to acrylic. It will add no structural support to the tank. It can be used as a gasket only on tanks which have a solid frame constructed, and the glass installed in a window like fashion.
Silicone does however bond to glass, and will actually hold the panels together.
If you are going to construct the tank out of acrylic, use a product called "Weldon 4". This solvent will literally weld the acrylic to itself, and if done correctly, the seams of your tank will be just as strong as the acrylic panels.
IMO 3/4" acrylic may be a little over kill for the length and height you are planning. But if that's what you have, go for it. But the thicker the acrylic, the harder it is to get perfect bubble free seams.
As stated before, this is a pretty big first time DIY project
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You will need to build jigs to hold the panels in place, and the application and curing of the solvent must be done in a specific manner, in a set amount of time. As long as your willing to do your research first though, and take your time, you'll end up with a great tank and the pride of knowing you made it yourself
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
If you have any specific questions, just ask. There are plenty of knowledgeable people here.