[Lsufan]

All silicone says it isn't intended to be submergered underwater. That is why on a tank the most important part is the seams inbetween the panes of glass. The seals on the inside corners of the tank are there to protect the seams & keep them from water.

The rtv silicone is one of the strongest U can get. It is a adhesives & not just a sealant like some silicones. It will bond to the glass great but I don't think it will adhere to the acrylic any better then other silicones. When u silicone the box just imagine wedging the box in place with the silicone. I don't know if u have looked but the only place local to me that carries the rtv is grainger. U will probably have to order it online. Honestly, for what u are doing I think any of the dap aquarium safe silicone that u can get from the big box stores will be fine. I would get the dap aquarium safe silicone because u know it isn't a neutral cure silicone

Your beneficial bacteria will be fine for the week that u don't have fish in the tank, so that is not a worry unless u don't let the silicone cure before refilling the tank.

What size tank do u have? That looks like a really small overflow. I do think that on a tank that is already setupthe best option is a overflow that is held on by the bulkheads