What size of tank is this mate??
have you seen the nitrates spike at all yet? if not, then your still going through your cycle...If they have peaked, then your due a good 50% water change..
I would not add a clown fish to the tank, not with the eel, and would preferable only add a good sized tang....If adding the tang, then ensure there is plenty of nori available for it, as the tank is pretty clean from algae.
Diatom algae is a single celled algae which is common in new aquariums, it gives an overall brown coloured coating appearance, and will adequate flow and low nutrient levels, usually passes after a few weeks..
Niko