I use a 10X-20X eye loop. It's a Russian make, good quality, and was purchased cheaply from Harbor Freight Tools. It comes with a stand that allows me to place it against the aq glass and focus on the micro critters on the tank side of the glass. I also use it to examine things removed from the tank, but that's harder as I have to fix and illuminate the object.
I have a 5X illuminated magnifyer on an extension arm on the work bench. I've been handy for fish surgery, cutting on corals, and general examinations.
My nikon allows me to focus a short distance into the tank and get macro shots which are magnified.
What I'd eventually like to acquire is a "dissecting" microscope with an adjustable tray (X,Y,Z), adjustable illumination, zoom magnification up to at least 30X, and a camera adapter. IMO you need at least 25X to see the identifiying skeletal features of SPS corals.