Many months ago my tank use to look exactly like that. At the time i was having a diatom outbreak, so I added some Seachem PhosGuard to help get rid of some of the silicates. After having it in my filter for about a month, my tank was crystal clear, and the diatom bloom subsided... so I took the PhosGuard out.
A few weeks later, my water started to get cloudy again, so I put some more PhosGuard in, and in a couple weeks it was clear again... so ever since, I've just been keeping a bit of it in my filter (bought a big tub of it), and my waters been staying clear. (I'm no scientist, so have no idea why the phosguard helps.. maybe it has something to do with silicate levels, and maybe my tap has high silicates.. but I dunno cause I havent tested it)