High nitrate: how to lower it for real

Updated on August 26, 2026 · ~6 min read

Nitrate should stay under 25 ppm in freshwater and around 1-10 ppm in a reef. It climbs because it is the end product of the nitrogen cycle and nothing consumes it on its own: you lower it with regular water changes, less food, siphoning the substrate and fast-growing plants.

Nitrate is the only one of the three nitrogen compounds you live with. Ammonia and nitrite must sit at zero; nitrate necessarily accumulates, because it is the terminus of the nitrogen cycle and nothing, in a normal tank, makes it disappear by itself.

Which means the value on the test does not describe a fault: it describes how long it has been since the last water change and how much load the tank carries.

Which values to hold

Type of tank	Reference	Why
Freshwater community	under 25 ppm	fish tolerate it well, but above it you see algae and chronic stress
Planted freshwater	10-25 ppm	the plants need it: at zero they run into deficiency
Reef	~1-10 ppm	corals suffer even at low values
Fish-only marine	under 25 ppm	more tolerant than a reef

The planted case is worth labouring, because it is counter-intuitive: in a tank with many plants nitrate is not waste, it is fertiliser. Driving it to zero "for cleanliness" puts the plants into nitrogen deficiency — older leaves yellowing and breaking down — and with the plants stalled the algae take the vacant place. Zero is not better than ten.

Why it climbs

Nothing in a standard tank consumes nitrate. The biological filter produces it, it does not dispose of it: nitrate is its end product. So the concentration rises in step with:

• how much food goes into the tank — that is the source, everything else is a consequence;
• how many animals there are and how big;
• the time since the last water change;
• whatever builds up in the substrate and the filter, which keeps decomposing.

Then there is one cause that is missed almost every time: tap water. In many farming regions the tap delivers 20-40 ppm of nitrate. Anyone changing 30% of the water every week and still unable to get below 30 ppm is probably topping up with water that is already loaded. You find out in two minutes: run the test on the tap water, not on the tank. If the tap is high, the only way out is cutting with RO water.

The methods that work

The water change

It is the reference method, the only one that always works and works predictably. The arithmetic is simple: changing X% of the water lowers the value by X%, if the new water is at zero.

Current nitrate	20% change	30% change	50% change
40 ppm	32 ppm	28 ppm	20 ppm
60 ppm	48 ppm	42 ppm	30 ppm
80 ppm	64 ppm	56 ppm	40 ppm

From which the consequence people always discover too late: from 80 ppm you do not reach 20 with a single change. It would take changing 75% of the water at once, which stresses the animals through the swing in parameters. The right route is two or three changes of 30-40% a few days apart.

⚠️ If the new water already contains nitrate, the sum changes: changing 50% with water at 20 ppm takes you from 60 ppm to 40, not to 30. The value you converge on is that of the new water, not zero.

Less food

It is the most effective method and the least practised, because nobody wants to be told they overfeed. The rule: whatever the fish do not finish in two minutes is surplus food, and it will become nitrate. One fasting day a week harms no healthy adult fish.

Siphoning the substrate

Detritus, faeces and sunken food keep decomposing where they lie. Siphoning the substrate during a water change removes the material before it becomes nitrate, instead of diluting the nitrate afterwards. In a tank with a thin substrate or a bare bottom you do it at every change; in a planted tank only in the open areas, without digging among the roots.

Fast-growing plants

Plants consume nitrogen in order to grow, so the fastest ones consume more of it. Floating plants are the most effective of all: they grow with maximum light and atmospheric CO₂, and you remove them by the handful, taking away the nitrogen they have stored. For this to work you have to prune them and throw the prunings away: a plant that grows and then rots in the tank gives everything back.

The right filter, and a wrong one

A bigger filter does not lower nitrate: it produces more of it, because it oxidises better. What genuinely lowers it, in a marine tank, are the anaerobic processes — a deep sand bed, a refugium with macroalgae, or a denitrification system — where bacteria reduce nitrate to nitrogen gas, which leaves the water. In freshwater the practical equivalent is plants.

Nitrate-removing resins do work, they are expensive and they exhaust. They are fine as a temporary measure, not as a system: if they are needed permanently, the problem is the load.

What not to do

• Do not change 90% of the water at once to "sort it out today". The swing in temperature, pH and hardness does more damage than 60 ppm of nitrate.
• Do not drive it to zero in a planted tank. The plants need it.
• Do not count on bottled bacteria to lower it: the ones sold for start-up are nitrifiers, meaning they produce it.
• Do not ignore the tap water. Until you measure it, you are working blind.
• Do not use test strips to decide: on nitrate they are particularly imprecise, and the difference between 25 and 50 ppm — the one you decide on — is exactly where they go wrong.

The indirect signal: algae

Nitrate high for a long time, often together with phosphate, is the condition in which algae thrive. But the relationship is not mechanical: there are tanks at 50 ppm with no algae and tanks at 10 ppm overrun. Light, movement, CO₂ and maturity count too. Lowering nitrate is part of the solution to an algae problem, not the whole of it — and raising it in a starved planted tank sometimes, paradoxically, reduces them.

How Gurglee does it

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