Spray Cleaner (I used Windex)
With a damp Magic Eraser carefully rub the water stained area of the wall, this will help loosen the muck as Magic Erasers are basically really fine spongy chunks of sandpaper. While the walls are still damp mist on some of the spray cleaner (avoid spraying over fish tanks and near electronics if you use Windex) over the area you just scrubbed (be sure it's a cleaner that won't strip off paint), and wipe off the area with a dry rag. If your water stains have been on your walls for a long time then you may have to repeat this process again another day, but most of it should come off the first time around.
Happy Fish Keeping!