Nitrates are all part of a grand cycle that should be happening in every aquarium that contains fish, plants, corals, or even just live rock. The Nitrate is the final stage of converting toxic waste into something that is less harsh for aquarium creatures to live in. In short, when food and fish waste is present in an aquarium environment, it starts in a toxic form of Ammonia. With the proper filtration and water conditions, nitrifying bacteria cause toxic Ammonia to be converted into a less toxic form of Nitrites, and then even into a less toxic form of Nitrates. This nitrifying bacteria plays a major role in the aquarium hobby. This process is known as Denitrification and it is done with biofiltration in the aquarium. 

Picture Source:

Shrimp Tank Cycling

More information on the Nitrogen Cycle and cycling methods:

Nitrogen Cycle Basics

This video might be helpful too: 

Keeping a proper level of Nitrates in an aquarium is important for fish organ health and growth of aquarium plants or corals.

As a very general recommendation, here are the nitrates levels required for different aquarium types.

Freshwater Fish Only- Under 40-50ppm

Freshwater Planted- 10-20ppm

Saltwater Fish Only- 20ppm

Saltwater Reef Tank- Under 5ppm

Nitrates above 100ppm have been proven to cause on-term damage of a fish internals. This high level cause a fish organs to have to work much harder than they are suppose to be. This causes unexpected death, with no real sign of the cause. A fish that is suppose to live 10-12 years, will maybe live only 4-6 years.

The Nitrates of the Nitrogen Cycle should also be the hint to do an aquarium cleaning and water change. If the Nitrates are monitored, an aquarium keeper will know how far to space out their aquarium water changes. Once the Nitrates climb above a desired levels for the aquarium that is being kept, this should prompt an water change.

For information on how to aquarium water changes, please see:

Resource for aquarium cleaning- Link

Nitrates can be considered dangerous a something around 30-40ppm. These numbers come from human studies. For drinking water, it is required to have these levels below 10ppm. For the higher levels in a fish aquarium, they could cause respiration issues, blood circulation issues, lowering the chance of breeding, lower activity, and lower disease resistance. Nitrates in the bloodstream can actually be converted back into Nitrites causing an issue known as "Brown Blood Disease".

High Nitrates also cause excess algae growth. Whatever remaining Nitrates that are left after the Nitrogen Cycle Denitrification, these can be used up by plants and algae. If there is more than the plants can keep up with, algae will take advantage of this and thrive off the extra Nitrates. Higher Nitrates can also create a excess of Nitrate Snails.

In a marine aquarium, higher Nitrates can stunt growth of corals or even cause enough harm to kill off the coral. A Cephalopods, something like a Octopus, these higher Nitrate level cause even more harm.

The causes of to high Nitrates would be poor biological filtration in the tank, high bio-load or fish for the filtration, over feeding, too long of time between water changes, plants decaying or other decaying matter.

There is a little less of a concern when it comes to too low of Nitrates. Since different aquarium require very low amounts, this is usually not an issue. It is ok to keep some of these aquariums at lower than recommended levels, but it is best to stick to the recommended level. These levels have been proven to be beneficial for certain aquariums.

What can become an issue is not having enough Nitrates for plant growth. Plants do need some Nitrates to feed off of. Corals have been found to do best in lower Nitrates, in almost zero conditions, so this would be a case, where there are no conditions when Nitrates would be to low. Nitrates just need to be focused on to know what level to maintain at.

One concern can be going from too high of Nitrates, to too low of Nitrates. This sudden extreme adjustment in water parameters will be to stressful on fish and could cause harm or death. All adjustments made to aquarium water should be done slowly and precisely. The same concern can be said for Nitrates jumping to quickly. If this happens, search out the cause in the sudden Nitrate jump. Also check Nitrites and Ammonia.

Water Changes- Nitrates are the aquarium keepers BIG hint that it's time for a water change. Try to maintain Nitrates by water changes. The best water changes are doing around 25% when Nitrates are passing the intended level. If Nitrates are too high for whatever reason, a aquarium keeper can do a 60% water change and only fill the aquarium back up to 80%. This will cut Nitrates in half!

Also keep in mind the Nitrate level of your tap/well water. This will determine how much Nitrates will be cut by the water changes. If the tap water read 25ppm and you do a 50% water change of water that has 100ppm Nitrates. This would make the Nitrates drop to 50ppm.

Plants- Plants use up Nitrates, so the more plants for marine or freshwater, the more they will maintain the Nitrates. Some plants do better than others. There are even algaes grown for marine tanks to help maintain Nitrates. Some plants also even remove Ammonia directly and the roots remove Nitrates and other Nirogenous waste near the roots in the substrate.

Vacuuming- Removing mulm build-up before it even starts the Nitrogen Cycle helps greatly. This can be done during the water change. There are also battery powered aquarium pumps, which allow an aquarium keeper to vacuum the aquarium ground, without removing water from the aquarium. This can be used for people who wish to extent the time between water changes.

Fish Load- Sometimes lowering the amount of fish which are in the aquarium is the only way to maintain safe Nitrate levels. The food and waste produced is just too much for the filters to handle. Keep less fish is an option or upgrade filtration. Also, make sure all filters that are already on the aquarium are being maintained properly. These need to be cleaned often to lower the mulm build up in the filter.

Bio-Load/DOC- Organic mulm/sludge turn into Dissolved Organic Compounds (DOC). This needs to be removed from the aquarium ground via vacuuming and filters. Clean filters weekly and if possible, have more than one form of filtration. Clean one filter one week, then the next filter the next week. Switching off ensures a high amount of nitrifying bacteria will still help keep the aquarium cycled.

Feeding- Watch the amount that is being feed to the fish. The food that is not consumed will turn into Ammonia and so will the waste of the fish after eating the food. Feed 1-3 times daily, and in a amount which can be eat around 3 minutes. It is hard to starve fish, if the aquarium keeper has them on some type of feeding schedule. It is very easy to over feed eat as they will just eat to eat. If the aquarium has high Nitrates and are unable to maintain low values, consider checking the food being feed. Many aquarium keepers over feed their fish.

Live Rock, Refugiums, or Protein Skimmers- These are Nitrate removal methods more geared towards marine tank owner. Consider adding more live rock, a refugium, or a protein skimmer for this type of aquarium. A saltwater aquarium is required to keep much lower Nitrates levels, so these methods are addition methods if needed.

High Porosity Bio Media- Rock that has high porosity will allow for deep pores of the rock to collect anaerobic de-nitrification bacteria. This is the good bacteria which keeps the Nitrogen Cycle going in the tank. Consider adding small chucks of live rock to filters, volcanic rock, or even SeaChem Matrix. This form of filtration has been proven to be very effective. These allow for de-nitrification deep inside these pores rocks. This is a little bit slower process to remove Nitrates, but long term is a much more stabile way to do it.

Other- There are handfuls of other man-made sold products to help maintain Nitrates. If all other of these methods listed above do not work, consider the use of: Nitrate "sponges", Zeolite, API Bio-Chem Zorb, SeaChem Purigen, Algone, NPX BioPlastics, Algae Scrubbers, and Micron Poly Pads. All of these recommends are based off professional used in the aquarium industry.

Resources:

Resource for Nitrate removing products- Link

What to do for High Ammonia and Nitrites- Link

The Nitrate levels of an aquarium are the window into seeing how well an aquarium is being maintained. The difference between someone that just takes fish and add them in water versus a person who is truly trying to create a suitable long-term environment for the fish to live, is how they maintain their Nitrates level. Consider the recommend Nitrates level for the aquarium which is being kept and use water changes to keep the Nitrates in these levels. Water Changes should be the first action to manage Nitrates. There are other methods if need be. The fish in the aquarium will say a big thanks for maintaining Nitrates.