Window 2100 x 780 x25 mm

Hi just looking at a window that’s for sale . It’s 2100 x 780 x25 mm My thoughts are. My ponds length internally is 3200.. if I left 100mm either side for the window to bond it would leave me with around 650 either side of the window.. or block and a half. Question is would this take the strian weight of water wise or do I need more block either side… ie smaller window ?? I was going to grind out a Chanel for rebar in the corners to assist ??