Get a better look at your livestock.  As fish swim into view, you can study them for any parasites. Investigate what is in your substrate, or on the rockwork. Enjoy those tiny polyps on corals. Objects need to be within range of the magnification, of course. Focal range is approximately 3-8", but I've been able to study things touching the inside glass easily.

The Flipper DeepSee Magnified Viewer is held in place magnetically, fitting glass up to 5/8" thick. Move it where you need it most. Safe on glass or acrylic.

Comes in three sizes: 
DeepSee Nano (3" diameter)
DeepSee (4" diameter) for up to 5/8" thick glass
DeepSee Max (5" diameter) for up to 3/4" glass

It's great for photography too!