Microscope advice
You do not need to spend much. You do need something that magnifies.
I recommend that everyone who keeps a marine aquarium buys a magnifying device. Relative to the cost of maintaining an aquarium it is very cheap, and it is useful for checking corals for pests, identifying nuisance algae, and examining fish for parasites. A microscope also opens another window into enjoying the ecosystem you have built.
Buying or borrowing oneHand lens or microscope, what to look for used, and who to ask if you would rather borrow one.
Setting it upThe parts of a microscope, getting the light right (Köhler illumination), and focusing tips.
Samples, slides, and picturesCollecting a sample, making a slide, and taking photos and video through the eyepiece.
Want to be convinced?
This video from Reef Rookies makes the case well. The microscope he buys is new. For the same money you can usually get a much better used one (see buying), but I have been telling aquarists to get a microscope for more than fifteen years, so it is nice to see someone else on YouTube making the same argument.