Though in smaller reef aquariums, the organisms in live rock and sand process Nitrate, reducing it naturally to zero. In larger reefs, that is over 90 gallons in size, with more life in, filters can help in keeping it at zero
However, there are several types of filtration that can help keeping your water quality high.
First and most versatile are Canister Filters. Make sure the filter or better filters are rated for both marine and freshwater aquariums.
By their construction, they can hold much media. What I use in mine is a cut-up and bagged Poly Filter, and Boyd’s Chemi-pure Elite. The reasons are, the poly filter removes whatever is in excess in your tank, and turns a color based on what it is removing. The Boyd’s is a clarifier and its resins remove excess Phosphate and Silicate. Why I always use those two kinds of media in the canister filters attached to my reefs is because in concert they continually clarify and remove dissolved undesirables from my reef water.
Essential, since I keep more reef fishes than is the norm in my tanks. I attach my canister filters to my sumps, since it keeps them out of sight and makes it easier to remove them for cleaning. Also, in-line heaters can be spliced into the outflow tubing of canister filters, also removing them from sight.
One must use aquatic gloves when cleaning a canister filter in used aquarium water, as bristleworms can proliferate in large numbers, particularly in the sponges, and the bristles can severely irritate your bare hands. Some people are allergic to those bristles. Also, you may be surprised by other kinds of marine life living in your canisters.
Similar to reactors in a way are Fluidized Bed filters.
What they are is acrylic towers, usually a good deal wider than the edges. The media is sand. A pump supplies tank water in sufficient flow to hold the sand in suspension.
Because of the weight of larger Fluidized Bed filters one may have to build a box or similar for it to sit on to reduce the stress of it hanging on the back of your tank.
Nitrifying bacteria proliferate on the sand grains, and are continually bathed in Oxygen-rich tank water. Fluidized Bed filters are very efficient in removing Nitrate from a fish only, fish only with live rock, and more heavily stocked reef aquariums. Because of the efficiency of the bacteria, the Nitrate is removed and the waste is pure Nitrogen gas.
You should know that if power goes out, when it returns Fluidized Beds can become Nitrate factories until the bacteria recover, which can take several days.
Fluidized Bed filters can be very helpful especially in tanks with larger marine fishes. The same is true in freshwater aquariums, since Fluidized Beds work just as well there.
Though by their very efficiency have fallen out of favor, for many years Wet-Dry filters were popular in marine aquariums.
In reefs, however, not so much.
Wet-Dry filters operate by fibrous media that is exposed to air, and moistened by slowly flowing tank water. The bacteria that proliferate on the media are extremely efficient, almost instantly metabolizing Ammonia and Nitrate. The result is Nitrate, which as quickly as the bacteria work, can become a constant problem.
And high nitrate can be a disaster in a reef aquarium, stressing corals to their ruin.
So, I recommend canister filters for mechanical reef filtration. Media can be placed in them to remove whatever you wish to from your aquarium water.