First of all, how the hell did you take a picture of something that small? The little dealies I have are only about a millimeter long, if that. Impressive. So what's the difference between amphipods and copepods, and are they beneficial?
Basically the same thing, as in small crustacean type animals. I believe Amphipods are bigger. The Amphipods tend to "scurry" around a bit more i find. Really creeping looking things and can freak you out, but totally harmless and good for your tank. Lots of fish eat them for food.
You think these look bad you should see the things that live in your bed......lol....
These don't hold a candle to dust mites etc. These just look like miniature microscopic shrimp. The things that creep me out are things that crawl over your hand that you can't see while you are reaching deep into your tank to retrieve something, usually bristle worms or such.......hair standing on end on my neck as I think about it.....
Or when you pick up a frag disc after sitting on the substrate for a while and grab under it and get a hand full of crawlies like bristle worms and pods racing around your hand looking for a new hiding spot.