Originally Posted by golfnut2
Will a half inch of sand be enough to remove the nitrates plus algae to help with phosphate or will 3.5 inches of space be enough to grow algae?
a half inch of sand will not support anaerobic bacteria to remove nitrates. You'll need t go with a deep sand bed for that, as onefish and pasfur suggested, 4-6", no less. 4" is a good depth though, I don't think there's a great need to go up to 6". A 1/2" sand bed is pretty much for looks only, if you don't want to go bare-bottom. The important thing is that it is not deep enough to really trap significant amounts of waste. Sand depths less than 4" aren't deep enough to grow de-nitrifying bacteria, but anything over 1/2" (but less than 4") will cause water quality problems by trapping organic wastes and letting them break down into nitrates without being removed by your protein skimmer.
As far as algae goes, it depends on what type of algae you want to grow. Assuming you're talking about macroalgae, there are several species, and many don't need a sandbed to grow. Chaetomorpha (or chaeto for short) is a very commonly-used macroalgae for sumps, and does not (to my knowledge) require a sandbed.
I guess my point regarding algae is, your choice of sandbed might not impact your choice of algae. It depends on what type of algae you want. But, since you want denitrifying benefits from your sandbed, you'll want to go 4"-6" of depth, which I think would be fine for any type of algae that does require sand to grow in.