Hi and welcome to the planted forum. I've just seen your whole tank in the Pictures forum - very nice too. I've just help a friend of mine set-up a Rio 240 - he's going the heavily planted route.
From the photo it looks more like a hair algae than diatoms. Algae is very common in new set-ups as the tank hasn't reached a biological balance. In these conditions algae tends to "win" against plants. You may be lucky and find that the algae will disappear on its own as the tank environment stabilses and your plants grow better. However, I fear that you may lack the necessary planting density for this to happen in the near future.
One drawback of the otherwise excellent Juwel aquariums is the lack of light. You will have 2 x 42" 38W tubes I presume giving about 1 WPG. Not many plants will grow that well in this light. Exceptions are some Crypts, Anubias, Java fern and moss. You will stand a much better chance of success if you use reflectors and change the supplied bulbs to full-spectrum.
For a more in-depth look at algae read the pinned topic at the top of this forum. Feel free to ask anymore questions. Hope this helps.
