How to seal a hole in an aquarium?

[Shaolin]

I have recently acquired a fish tank of approx. 200L. One corner has a circular hole about 2" in diameter that had a pipe going out from it. I want to seal this hole. I have some 3mm acrylic sheet and some aquarium-friendly silicone sealant, so questions:

1 - Is the acrylic ok to use? Should I be using thicker stuff?
2 - Do I use the silicone like an adhesive between the acrylic and surrounding glass or place a square onto the glass and just seal around the edges?

[Shaolin]

Thanks for your replies, a sump isn't possible due to positioning, neither is having anything sticking out from the underside, I want the bottom to be flat. If I seal as I suggested, I could easily cut it out in the future if I wanted to, no?