If you really must have a mirrored look, I would suggest using mylar film (automotive parts store). This is totally plastic & inert, so you can just glue (silicone) it to the area. You may also consider some other mylar finish other than polished mirror ... maybe a smoked mirror look ... that is not so reflective so the fish may be less spooked.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.