[0sprey]

If all you want is softies, the lighting you've got should be fine. You'll see some interesting stages of algal growth, and if you're lucky, you may even find some macroalgaes sprouting from the rock.
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If you aren't keeping stony corals, don't worry about calcium. Seriously. You won't have any heavy calcium consumers, so regular water changes should be more than adequate to keep calcium, magnesium and alkalinity balanced. That's the most important thing- don't skimp on the water changes, especially in a nano like the one you're running.

A protein skimmer is a real boon to a tank. They remove all kinds of dissolved organics-once you've seen what a good skimmer can pull out of your water, you won't want to go without one! Buy the best you can afford. The usual recommended HOB skimmers (I'm assuming you don't have a sump) are the CPR BakPak, the AquaC Remora, and the Deltec MCE series. Most people would agree that the Remora is probably the best 'bang for your buck'. Do some searches online, read the reviews, and decide which one you want. Don't just settle for what's readily available- it's worth it to order a better skimmer online if you can't find it in the store.

Nothing wrong with a HOB filter... but ditch the bio-media. Just use it to run chemical filtration (carbon, phosphate remover), and make sure you replace it every month. Then you don't have to worry about organics building up and causing problems.