August 27, 2026

Reef tank parameters: the target chart, and which number to fix first

You have just finished a full round of tests and three numbers are out. Alkalinity is 6.4 dKH, salinity reads 1.023, nitrate is 35 ppm, and every tab you have open gives a different target. One quotes natural seawater. One quotes what a coral farm runs. One says phosphate should be zero.

Here is the short version. There is a narrow set of reef tank target ranges that almost everyone experienced agrees on, and it is in the chart below. What the charts never tell you is the order to fix things in when more than one is off, and that order matters more than the exact numbers. Get it wrong and you spend a month chasing your own corrections.

The reef tank parameters chart

Acceptable is where corals still do fine and you do not need to act today. The last column decides how often you test and how fast you have to react.

What each row means in practice

Temperature. Stability matters more than the number. 78 F (25.5 C) held all year beats 77 F (25 C) in winter and 82 F (28 C) in August. If you are wondering about the top end, what temperature should a reef tank be (and is 82 too hot) goes through it.

Salinity. Natural seawater is 35 ppt, which is 1.026 SG at 25 C. Whether to run 1.025 or 1.026 is mostly about consistency. What matters more is whether your instrument is honest: a refractometer calibrated on RO water instead of a 35 ppt reference can read 0.001 to 0.002 out, and a swing arm hydrometer is worse. Which salinity tester you can trust covers the options.

pH. The daily swing is bigger than most of the range, so one reading tells you almost nothing. Test at the same hour every time. Low pH with normal alkalinity is a CO2 problem in the room, not a chemistry problem in the tank, which pH vs alkalinity explains. If it sits below 7.9 at its evening peak, how to raise pH in a reef tank has the fixes that work, and buffer is not one of them.

Alkalinity. The one that moves every day and hurts corals fastest when it moves too far. Natural seawater is about 7 dKH, and 8 to 9 gives you a margin against a missed dose. Above 9 is fine with nutrients to match and risky in a very low nutrient tank, where 11 dKH plus near zero phosphate is the classic recipe for burnt tips on SPS. Raise it by no more than 1 dKH per day, and how to raise alkalinity in a reef tank covers the products and the maths. Your daily consumption is the number to know: how to calculate daily alkalinity consumption shows how to measure it, and the alkalinity consumption calculator turns two readings into a daily figure.

Calcium and magnesium. Slow movers that rarely cause acute problems, but they set the conditions for alkalinity to behave. Low magnesium lets calcium and carbonate precipitate together, so alkalinity refuses to hold however much you dose. High calcium is nearly always dosing out of balance rather than a problem in itself, which how to lower calcium in a reef tank covers.

Nitrate and phosphate. Zero is not the target. Corals need both, and held at undetectable levels they lose colour and tissue. Above 20 ppm nitrate or 0.2 ppm phosphate, growth slows and algae gets a foothold, and green hair algae is usually the first symptom. Nitrate comes down with export rather than chemicals, and how to lower nitrates in a reef tank has the order to try things in. If you carbon dose, start at a fraction of the recommended dose and step up over weeks, because a full dose into a nutrient rich tank can trigger a bacterial bloom that strips oxygen overnight. Lanthanum chloride for phosphate must go through a filter sock or reactor and never straight into the display, because the precipitate lodges in fish gills.

Ammonia and nitrite. Zero in an established tank, and only worth testing after a death or an over enthusiastic filter clean. During a cycle they are the only two numbers that matter, and nitrite sitting at the top of the chart for weeks is normal. Nitrite not dropping to zero covers the cases where it is not.

ORP. Most healthy tanks sit between 300 and 400 mV and drift 30 or 50 mV a day. Treat it like a smoke alarm: a slow decline over a week means something is decaying or the skimmer needs cleaning, a sudden drop means go and look in the tank now. Never dose anything, or run ozone, to hit a specific ORP number.

Which reef parameter to fix first

Ordered by how fast each parameter can hurt livestock, not by how far it is from target.

1. Ammonia and nitrite, in a young tank or after a death. Ammonia at 0.5 ppm at reef pH is already stressing fish. Water change, bottled bacteria, stop feeding, find the source. Nothing else here is worth touching until ammonia reads zero.

2. Temperature. A tank at 84 F (29 C) needs a fan and the lights off this afternoon, not next week. Bring it back over a few hours, a degree or two an hour, because the swing itself is what stresses livestock.

3. Salinity. No more than 0.001 SG per day, so a tank at 1.021 takes five days to reach 1.026. Raise it by topping up with mixed saltwater instead of RO, lower it by swapping tank water for RO, both into the sump where there is flow. In a 200 litre (roughly 50 gallon) tank, about 8 litres swapped for RO drops salinity by roughly 0.001 SG, one day's worth. Never add dry salt to the display.

4. Alkalinity. First of the chemistry parameters because corals feel it within a day. Raise it at no more than 1 dKH per day, into the sump or a high flow area and never straight into the display where it can settle on a coral before it mixes. If it is high, stop dosing and let consumption bring it down rather than adding acid. If magnesium is below 1250 ppm, fix that first, because low magnesium is why alkalinity will not hold.

5. Calcium. Only after alkalinity has held for two or three tests. The two are consumed together, roughly 7 ppm of calcium per 1 dKH, so dosed in that ratio they tend to stay in range together. Reef calculators explains how the two products relate and how to work out a dose.

6. Nitrate and phosphate. Out of range for weeks already, they can stay there a few more. Fix them last, one lever at a time, after everything above is steady, because most nutrient fixes (carbon dosing, GFO, a big water change) also move the numbers above, and you want those steady so you can see what the nutrient fix actually did.

7. pH. Almost never fixed on its own. With alkalinity in range, low pH is CO2 from the room, and the fixes are ventilation, or a CO2 scrubber on the skimmer intake. Kalkwasser as your alkalinity source raises pH as a side effect, but it must be dripped slowly or run through a doser and never dumped, because a slug of it spikes pH past 8.6 and precipitates calcium carbonate across the tank.

8. ORP. Never on the list. It recovers when whatever is above it is fixed.

Why one thing at a time beats fixing everything at once

Every correction moves at least one other number. An alkalinity dose raises pH for an hour. A 25% water change moves salinity, temperature, alkalinity, calcium, magnesium and nutrients at once, by an amount that depends on what is in the bucket, which does your reef salt mix change your parameters goes through brand by brand. Kalkwasser raises alkalinity, calcium and pH in one go.

Now imagine fixing three things on Saturday morning: a big water change, a double alkalinity dose and a fresh bag of GFO. On Sunday the corals are closed. Was it alkalinity moving 2 dKH in a day? Phosphate dropping from 0.15 to 0.02 ppm overnight, which SPS hate? A salinity mismatch in the new water? You cannot know, so you cannot fix it. Corals not opening after a water change covers exactly this situation.

The alternative is boring and it works. Fix the top item. Re-test in 24 hours for temperature, salinity and alkalinity, in a week for calcium, magnesium and nutrients, and move on only once the number has held for two or three readings.

Two or three, not one, because hobby kits have error bars: about 0.1 to 0.2 dKH on alkalinity and 10 or 20 ppm on calcium. One reading 0.3 dKH below yesterday's is noise. Three in a row heading the same way is a trend. Adjusting after every reading is how tanks end up swinging between 6 and 11 dKH.

Ideal is a band, not a number

"Ideal reef tank parameters" is a phrase that slightly misleads. Natural seawater runs about 7 dKH, 420 ppm calcium, 1280 ppm magnesium and phosphate below 0.01 ppm, and a tank held at exactly those values with normal hobby feeding usually looks worse than one at 8.5 dKH and 0.05 ppm phosphate, because the tank is not the ocean.

Every list of saltwater aquarium parameters differs at the edges, and that is fine. The chart gives you a band a healthy mixed reef can sit anywhere inside. Pick a target inside it, write it down, and hold it. A tank at 8.0 dKH every day for a year grows more coral than one averaging 8.5 while wandering between 7 and 10. Stability inside the band is the goal.

Write the targets down

The chart is the general answer. Your tank has a specific one: whatever combination your corals have been growing in for the last six months. The only way to catch a parameter leaving it before the corals tell you is to log readings against your own targets somewhere you will actually look.

That is what the free parameter tracking spreadsheet is for. Set your bands on the Targets sheet, enter readings, and the cells turn green or red against your numbers. Pair it with the testing schedule, which sets how often each row above needs testing, and you have the whole system.

Get the spreadsheet
Make a copy in Google Sheets (needs a Google account)
Download the Excel version (no account needed)

Free, no email, no attribution. Readings turn green inside your target band and red outside it, and the charts draw your target band as dashed lines.

Holding eleven parameters against eleven target bands, each on its own test cadence, is a lot of bookkeeping to carry in your head or a notebook. Recif is the reef companion app we built to do it for you: it logs every test, charts each parameter against your own target ranges, works out consumption trends, reminds you when a parameter has gone too long untested, and rolls the whole tank into one Stability Score so you can see at a glance which number to look at first. It is available on both the App Store and Google Play.