Aquarium pH: ideal values and how to correct it
Updated on August 26, 2026 · ~6 min read
A freshwater community tank is fine between pH 6.5 and 7.5, a reef between 8.0 and 8.4. The exact figure matters less than stability: correcting pH with «pH down» products causes swings more harmful than the wrong value. You work on KH, CO₂ and water changes instead.
pH is the parameter people talk about most and get wrong most, for a simple reason: it is easy to measure, so it becomes the number you want to act on — while it is mostly a consequence of other parameters rather than a lever.
What it is, in two sentences
pH measures how acidic or basic water is, on a scale from 0 to 14 where 7 is neutral. The scale is logarithmic: pH 6 is ten times more acidic than pH 7, and a hundred times more than pH 8. That is why a jump which looks small on paper — 7.5 to 6.5 — is enormous for a fish.
Reference values
| Type of tank | pH |
|---|---|
| Freshwater community | 6.5-7.5 |
| Soft water (discus, neon tetras, Caridina shrimp) | 6.0-7.0 |
| African rift lake cichlids | 7.8-8.5 |
| Reef and marine | 8.0-8.4 |
| Brackish | 7.5-8.2 |
These are ranges, not targets. Most farmed fish on sale tolerate a pH somewhat outside their ideal, provided it is constant; none tolerates a pH that swings half a point every few days. Between a steady 7.8 and a 7.0 that jumps to 7.6 every week, a soft-water fish lives better in the first.
Why you do not correct it directly
The "pH down" and "pH up" bottles work — for a few hours. Then pH returns where it was, because the tank contains a buffer that pulls it back to its equilibrium value, and that buffer is KH.
KH (alkalinity, or carbonate hardness) is the water's reserve of carbonates and bicarbonates: it is what absorbs acids and stops pH from moving. At 8 dKH pH is practically immovable; at KH 2 very little shifts it. Adding acid to water with high KH is a tug of war: you consume the buffer without moving pH, until the buffer runs out and then pH collapses all at once. That is the mechanism of a pH crash, and it kills a tank overnight.
So the practical rule: you do not correct pH, you choose your water. Anyone who wants a stable low pH starts from water with low KH (cut with RO, peat, wood, catappa leaves) and has it for good. Anyone who wants a stable high pH keeps KH high. Anyone with average tap water and community fish should not touch anything.
Before correcting anything, measure KH. A "wrong" pH with normal KH is often not a problem. A correct pH with KH near zero is a tank one step from collapse, even though the number you are looking at is the good one.
Why pH changes over the day
In a planted tank pH swings by nature, and that is not a fault. During the day plants consume CO₂ for photosynthesis: less CO₂ means higher pH. At night they respire and produce it: pH falls. In a planted tank with CO₂ injection the difference between morning and evening can be 0.5-1.0 points.
Two practical consequences follow:
- Always measure at the same time of day, otherwise you are comparing one piece of data with a different one. The most useful moment is the morning, before the lights come on: it is the daily minimum.
- Do not chase the natural swing. That is the tank breathing. What matters is the trend week over week, not the difference between two readings eight hours apart.
In marine tanks pH also falls for another reason: the CO₂ in the room's air. A tank in a closed, occupied room can sit half a point below the same tank with the window open. If a reef's pH is low but KH, calcium and magnesium are in order, the first suspect is the ventilation of the room.
How to move pH for real
To lower it, stably:
- Cut with RO water, which lowers KH and GH and so leaves pH free to fall. This is the method that always works.
- Wood, catappa leaves, alder cones, peat in the filter: they release humic acids and tannins, lower pH gradually and tint the water amber. It is the natural environment of many soft-water fish.
- CO₂ in a planted tank: raising its concentration lowers pH predictably. With injection you sit at 20-30 ppm of CO₂, in a low-tech tank at 3-8 ppm. ⚠️ If the fish stay at the surface with their mouths open, shut the CO₂ off and increase movement: the animals' safety comes before the plants.
To raise it, stably:
- Add KH with baking soda, in very small doses: about 3 g per 100 L (26 gal) raises KH by 1 dKH. No more than 1 dKH per day — a fast swing in KH does more damage than a value slightly out of range.
- Calcareous rock and substrates (aragonite, coralline): they buffer upwards permanently. That is how you hold African cichlid water without dosing anything.
- More surface movement, which helps gas exchange and drives off excess CO₂.
pH and ammonia: the combination to know about
There is one case in which pH stops being one parameter among others and becomes urgent: when there is ammonia in the tank. Ammonia exists in two forms in equilibrium, NH₃ (very toxic) and NH₄⁺ (nearly harmless), and pH decides the proportion: the more alkaline the water, the higher the toxic fraction.
Practical consequence: with ammonia in the tank, do not raise pH. Anyone who "fixes all the parameters at once" during an emergency can convert the harmless part into the toxic one in a single move.
How to measure it
A liquid test kit is fine and cheap; test strips read pH coarsely but legibly. An electronic pH meter costs little and reads decimals, but must be calibrated with reference solutions every few weeks: an uncalibrated instrument gives three wrong decimals with great confidence, which is worse than a liquid test read by eye.
Three frequent mistakes: reading the colour under artificial light (read it in daylight, against a white background); using the "high pH" test when you need the "low pH" one or the other way round, because each kit only covers part of the scale; comparing readings taken at different times of day.
How Gurglee does it
Gurglee tracks pH together with KH and CO₂, because they are the same question seen from three sides, and for every parameter it shows the reference band for your type of tank. On pH the app does not offer the water-change simulation, and that is deliberate: pH is logarithmic and governed by KH and CO₂, so a dilution formula would give a precise, wrong number — the app writes why it cannot be estimated instead of estimating badly. The charts show the trend against the target band, which is how you tell the day/night swing from a real drift.
Related guides
Water hardness: GH and KH explained simply
GH and KH are not the same thing: what they measure, which values fish and shrimp need, and how to raise or lower them without doing damage.
Ideal water parameters chart by type of tank
The reference values of every water parameter, for freshwater, planted, brackish, marine and reef tanks, in one chart you can come back to.
Ammonia in an aquarium: why it is the most dangerous
What ammonia is in an aquarium, why it kills, where it comes from and what to do when the test detects it. With the role of pH and temperature in its toxicity.
How to read a liquid test kit without getting it wrong
Liquid test kits go wrong less often than the people reading them: light, background, angle, waiting times, expired reagents and the traps of each individual kit.