IMO, nitrates in a reef system is not good—any nitrates. You want to aim for zero or very close to it. And with a dsb that shouldn't be difficult.
I'd say you are way overloaded for a new system with 200 creatures. Algae is a natural process for your newly cycled reef tank and a vital food source for all of the snails you have in the tank. If you try to get rid of the algae, including the diatoms, all of those snails will die. Keep the front glass crystal clear and let the side and back glass get covered. In a few months, as the turbos get large, they will keep your glass clean.
I will say that you really have too many crabs in there for a reef setup. All of those crabs will become a nuisance once they start growing and when you start trying to add corals in your tank.
For a true DSB system, you really just need LS [preferably 6” with a diversity of grain particles ranging from approx .02-1.7], LR, a protein skimmer and circulation pumps. You can add a sump to house equipment. That wet-dry is wasted equipment on a DSB, however, the added volume of water is always a plus. I would definitely take out the bioballs, and would not use carbon continuously. If you use filter pads, clean or change them weekly.
Are you aiming for a reef tank? It looks that way to me which is why I would not recommend the continuous use of carbon filtration or filter fine meshed pads for that matter. You are filtering out essential water-bound micro organisms by processing them thru theses filters.
You DSB needs epifauna and infauna organism. [creatures that live on and in sand] to establish that DSB [your primary source of filtration], not so many crabs, though [a few will be ok]. Starter kits with these organisms can be obtained from <a href="http://ipsf.com/" target="_blank">ispf</a> and <a href="http://inlandaquatics.com/prod/prod_detrit.html" target="_blank">detritivore kits</a> . While the turbo snails are fine, biodiversity is the key element to the establishment of a balanced, nearly self sustaining reef system. I would leave in a few crabs, and add a variety of species of snails, including the fighting conch. Worms are the name of the game for the your infauna and you need to populate the DSB with as much biodiversity in this area as possible.
But it all depends on what type of system you are aiming for. With the DSB you mention, I assume you are attempting to establish a reef system with a natural filtration of LR and DSB. If you are shooting for a different type system, then it would help us to know that so that we can give you the best advise.