The nitrate-to-phosphate ratio, and why your tank barely reads it

A mass ratio around 10:1 NO₃:PO₄ is a decent planted-tank starting point — it falls out of the old Redfield ratio — but the ratio is the least important number on the list. What keeps algae down is holding both values in a sane absolute band (NO₃ 5–25 mg/L, PO₄ 0.1–2.0 mg/L for a planted tank) and keeping them stable. A tank sitting at a "perfect" 10:1 with wild swings and too much light will grow more algae than a tank at 20:1 that never moves. The ratio is a symptom of a balanced tank, not the lever that balances it.

That's the whole guide in three sentences. The rest is why the ratio got its reputation, where it genuinely helps, and what to actually reach for when the glass goes green.

Where the 10:1 number comes from

The ratio traces back to Alfred Redfield, an oceanographer who noticed in the 1930s that marine phytoplankton — and the deep water they drifted in — held carbon, nitrogen and phosphorus in a remarkably steady atomic proportion: roughly 106 : 16 : 1 for C : N : P. The nitrogen-to-phosphorus slice of that, 16 : 1 by moles, is the bit the hobby latched onto.

Sixteen-to-one sounds nothing like ten-to-one, and that mismatch trips people up. The gap is just units. Redfield counted atoms; your test kit reads nitrate and phosphate as whole molecules, which are heavier than the bare N and P atoms buried inside them. Convert the molar ratio into the mass ratio you actually measure and it lands close to ten:

Redfield (by moles):     N  : P   = 16 : 1
Measured as compounds:   NO₃ : PO₄

16 mol NO₃ × 62 g/mol  =  992 g
 1 mol PO₄ × 95 g/mol  =   95 g

NO₃ : PO₄ (by mass) ≈ 992 : 95 ≈ 10.4 : 1

So a NO₃:PO₄ mass reading of about 10:1 is the freshwater translation of Redfield's 16:1. Some hobby sources quote 15:1 or 20:1 as the target instead, usually because they skipped the molar-to-mass conversion, or because they're padding the nitrogen side on purpose. None of these are laws of nature. They're one ocean chemist's average, reheated for a market Redfield never studied.

Why an ocean average doesn't fit a glass box

Redfield was describing free-floating plankton in the open sea, where the water is the organism's whole world and nutrients arrive dissolved and evenly mixed. Your tank is nothing like that. It has rooted stem plants and crypts that feed partly through the substrate, fish that dump ammonia and phosphate on a feeding schedule, a filter quietly processing nitrogen, and — if you dose — a bottle you tip in on your own cadence. The nutrients are neither evenly mixed nor arriving in Redfield's proportions.

Aquarium plants also don't hold nutrients in a fixed ratio the way plankton roughly do. They do luxury uptake: given plenty of a nutrient, they hoover up far more than they need right now and bank it for later. A healthy stand of stems will happily run at all sorts of internal N:P ratios depending on what you've been feeding it. And "algae" isn't one organism with one appetite — green spot, green dust, hair, staghorn, black brush and cyanobacteria all have different triggers and different tolerances. There is no single ratio that starves all of them at once, because they don't all eat the same way.

Transplanting a 1:1 rule from the North Atlantic into a 120-litre planted box is, at best, a loose analogy. At worst it sends people chasing decimals while the real problem sits untouched.

What actually grows algae

Strip the ratio away and the drivers of common freshwater algae are boringly consistent:

• Light — too intense, on too long, or too strong for how fast the plants can actually use it. This is the single biggest lever in most tanks.
• Instability — a new tank that hasn't settled, CO₂ that swings through the photoperiod, a dosing routine that lurches. Fluctuating conditions favour algae because plants are slower to adapt to change than algae are.
• Organic waste, especially ammonia — decaying food, mulm, an overstocked tank, a stalled cycle. Ammonia in particular is a well-documented trigger for algal blooms, which is one reason a new tank algaes up before it finds its feet. The nitrogen-cycle guide covers why that ammonia is there and how to make it go away.

Notice what isn't on that list: the exact ratio between two nutrients that are both comfortably present. When you read forum threads blaming a "12:1 instead of 10:1" for an algae outbreak, the tank almost always had a light or instability or waste problem doing the real work, and the ratio was a coincidence that got the blame. Fixing the ratio and leaving the light at nine hours of blazing intensity fixes nothing.

The bands that matter more than the ratio

Here's the honest hierarchy. Absolute level first, stability second, ratio a distant third.

For a planted community tank, aim to keep nitrate roughly in the 5–25 mg/L band and phosphate in the 0.1–2.0 mg/L band. Both are floors as much as ceilings — plants need measurable amounts of each to build tissue. If both numbers sit inside those bands and don't lurch around week to week, your NO₃:PO₄ ratio will typically land somewhere between about 5:1 and 50:1 on its own, and every value in that whole window is fine. You do not need to hit 10:1. You need to hit "present, and not swinging".

The ratio only becomes worth a glance when one side is scraping zero, because that's when a nutrient stops being a background number and starts being a hard limit on growth.

When one side runs to zero

This is the scenario where the ratio-people are accidentally right, even if their reasoning is off. It's not that a wrong ratio grows algae — it's that a nutrient at zero starves the plants, stalled plants stop competing, and algae walks into the empty seat.

Phosphate bottomed out. Push PO₄ to zero — easy in a lightly-fed, well-planted tank, or one running an aggressive phosphate remover — and plant growth stalls for want of phosphorus. The classic tell is green spot algae speckling the glass and the older leaves, plus stalled growth on plants that need phosphate to build it. The ratio here might read as a huge NO₃:PO₄ number, but the fix isn't "correct the ratio", it's "put phosphate back in".

Nitrate bottomed out. Run NO₃ to zero and plants yellow from the older leaves inward, growth thins, and the tank often invites cyanobacteria (the slimy blue-green sheet), which fixes its own nitrogen and thrives exactly where nitrate-hungry plants can't. Again the ratio is a side effect; the cause is an empty nitrogen tank.

The trap in both cases is the "limiting nutrient" idea taken too literally — the belief that keeping a nutrient scarce will starve algae. In a planted tank it starves your plants first, because they're the ones you're relying on to outcompete algae, and a stalled plant is worse than useless. Feed the plants; don't try to starve the tank.

Moving each side without chasing a decimal

If a value is genuinely out of band, adjust the offending side directly rather than reverse-engineering a ratio:

Goal	What to reach for
Raise NO₃	feed a little more; dose nitrogen (KNO₃ / an all-in-one)
Lower NO₃	water change; less food; more/faster plants; check stocking
Raise PO₄	feed more; dose phosphate (KH₂PO₄); ease off any phosphate remover
Lower PO₄	water change; less food; phosphate remover as a last resort

A few things worth knowing. Fish food is rich in phosphorus, so a well-fed tank rarely runs short of PO₄ — it's more often nitrate that bottoms out in a heavily planted, lightly stocked setup. If you dose dry salts, potassium rides along for free in both KNO₃ and KH₂PO₄, which is handy. And a water change moves both nutrients toward whatever your tap carries (usually near-zero for both), so it's a blunt way to knock a high number down — the water-change guide has the dilution maths for how far a given swap actually moves things.

If you'd rather work out a specific weekly dose for your volume and target than eyeball it, the dosing calculator does the arithmetic. And if you're running lean on a low-tech tank where carbon is the real ceiling, planted without CO₂ explains why heavy Estimative-Index dosing — the deliberate ignore-the-ratio flood — is the wrong tool without injection.

The honest takeaway

The NO₃:PO₄ ratio is a real thing with a real pedigree, and it's a fine sanity check: if you compute it and get something absurd like 100:1, that's a useful nudge that one nutrient has crashed. But it is not a dial you tune for algae control, and treating it as one is how people end up with a spreadsheet-perfect ratio and a green tank.

Log nitrate and phosphate as separate numbers, keep each in its own band, and watch whether they're steady — a flat, boring line is the goal, and it's exactly the shape the trend view is built to show. The ratio between two well-behaved numbers takes care of itself. The moment you find yourself dosing to fix a ratio rather than to fix a level, stop and check the light first.