Cownoseray,
I recently was contemplating doing the same thing. I happen to have two of those American Farmland 8x2' water tanks that I have used in to past to hold large fish temporarily. I don't believe that it would be possible to add a very large curved window to one of those tanks. However they do have 4 small flat sections to them that one could potentially add viewing windows too.
However, I have some concerns with adding viewing windows to these tanks. First I am almost certain that they are made from HDPE (correct me if I am wrong). As we well know HDPE is renowned for its inability to bond to various materials. So at the very least forming a mechanical bond with Dow Corning 795 will be difficult. Second, I am not sure how adding a viewing window to these tanks will effect their structural integrity. For all I know the tank may not be able to handle the additional stress and just fall apart.
Zoodiver is correct with the usage of bolts, nuts and washers for securing the window into place. The 795 will simply act as a gasket. I would recommend that you first make small silicon dampers 1/2" tall around the lip of the window. Instead of applying a beed of silicon to the lip of the window and then lifting it into location, I would recommend that the window already be placed into location with the bolts fastened down loosely. Then inject the Dow Corning 795 silicone into the gab between the window and the tank. This will ensure no air pockets are formed in the silicone seal and thus no leaks. After the 795 has reached full cure, tighten the bolts snugly.
I would strongly recommend prepping the surfaces with 60 grit sand paper before instillation. This will help the Dow Corning 795 achieve maximum adhesion.
The main reason I decided against preceding forward with this project was due in large part to my lack of available time. I may reconsider this project at a later date but right now it's not a possibility.
Best of luck and keep us posted if you go forward with this build.
