Sorry if this has been asked before but I have not made it through all the posts yet.
When I was thinking about your question I had a particular post in mind but after going back and reading it I see I really did not explain much about nitrate removal. The post I am talking about is here
So I will try to expand on where that post left off.
Nitrate is the the item that is produced when nitrite is broken down and processed. Nitrate is not toxic to fish for the most part but is toxic to some degree to corals. So in a fish only system nitrate levels of 50ppm - 80ppm I would say would be ok. Not desirable
but ok. In a reef setup with corals levels of 10ppm or higher are not desirable because they will cause corals to close up and not be as healthy.
Wet/dry filters are excellent ammonia and nitrite removers but poor nitrate removers simply because of their design. Nitrate removing bacteria requires low levels of oxygen to live where ammonia and nitrite removing bacteria thrive in highly oxygenated water. The trickling action of a wet/dry filter greatly enhances the oxygen content of the water thus the ammonia and nitrite bacteria thrive.
allows for those low levels of oxygen. Large chunks of LR
also allow this deep inside the core of the rock, a plenum system albeit not popular right now also served the same purpose.
With a sand bed of 4" or so and the sand grain being mostly composed of very small 'suger grain' sized particles the upper surface of the bed can hold the ammonia and nitrite removing layers of bacteria. Further down in the center or bottom of the sand be where water motion is almost non-existant the oxygen levels are very low. Its at these locations that nitrate removing bacteria thrive.
Also are phosphates a problem in saltwater tanks? If so how are these controlled?
Yes they can become a problem. Phosphates are basically fuel for algae to grow. usually the undesirable kind. You can help limit the phosphate level in your tank by using RO
water and also feeding spairingly. Many prepared foods actually have phosphates added.
Another method to control phosphates is to have lots of desirable algae in the tank. The theory is that this desirable algae will out compete the undesirable alage for the phosphates.
Keep the good questions coming.