I have also found that the magna floats do scratch your aquarium's glass if they pick up sand.
They must be cleaned in a vinegar solution after every few uses to remove any deposits that accumulate after use.
I only use my Magna Float to get to difficult to reach areas of the tank - the area where the top of the substrate meets the water and the corners of the aquarium are where the Magna Float really shines, since the algae here tends to grow heavier and can be more difficult to remove.
In fact, I will often just use the inner part of the Magna Float to scrub these areas by hand, instead of using both parts of this scraper, since it can be awkward to move in tight areas.
A razor blade or credit card can also be used in such areas. I use a Penn Plax brush with a long handle to clean the hair algae and diatoms which grow on the rest of the glass, since this works well, with less chance of scratching the aquarium glass.
The Magna Float is a great concept, as long as you take care to use it in areas where a scratch will not be noticed - ie the bottom areas of your aquarium.
For the rest of the tank a scrub brush on a long handle works best in my opinion.
If you are using high lighting for your corals and haven't already installed a uv sterilizer, you might consider doing so, since they do a good job of killing off algae spores which helps to prevent algae blooms.
