Ok - that particular "coin" microscope you showed is pretty much useless for reefing...
You need to get to a real actual 100 times magnification to 400 times magnification for identification of phytoplankton and such. That means you need a microscope that can actually show you true magnification and not just garbage digital zoom from an image projected on a digital sensor and then rendered on a small screen.
For a cheap, well under 200.00 usd pricetag, solution you can try finding an actual monocular compound microscope like the AmScope 150 series where your looking at 40x-1000x through the use of a 10 times eyepiece using the objective lens settings of 4x, 10x, 40x - letting you see 40x, 100x, and 400x / with the 25 times eyepiece, you get 100x, 250x, 1000x - however you loose the compounded viewing when you use a usb camera like the amscope wd200 series because it replaces the initial eyepiece and that removes the compounding view so you end up with a camera view 4x, 10x, and 40x and then need to rely on digital zooming in where your image ends up being only as good as the sensor is that the objective lens is focusing on. Do your research before you buy and understand what your actually buying :)