They're common if you have plants or natural decorations in the tank (such as that driftwood). I noticed in the pic you have plants. I have noticed that I NEVER have any diatoms until I decide to buy a plant. Then they emerge. Just take a brush, go in, and scrub it. I noticed you have them growing on your plants too. They will actually eat away your plants over time, so you don't want them on plants. On rocks, wood, not a major problem.