pH, KH and GH in a goldfish tank

Most keepers meet these three readings as three separate rows on a test kit chart, and treat them as three separate facts about the tank. They are not. Two of them are directly causally linked, the third mostly rides along, and understanding the relationship converts a confusing set of numbers into a single, fairly simple story.

The story is this. KH determines whether your pH stays where it is. pH determines how poisonous the ammonia in your tank is. GH is a measure of dissolved minerals that goldfish are largely indifferent to. Everything below is detail on those three sentences.

The testing guide covers how to take the readings and what the target ranges are. This page is about what they mean.

pH: a state, not a substance

pH measures how acidic or alkaline water is, on a scale where 7 is neutral, lower is acidic and higher is alkaline. The critical property, and the one most often missed, is that the scale is logarithmic: each whole number is a tenfold change. Water at pH 6 is ten times as acidic as water at pH 7 and a hundred times as acidic as water at pH 8.

That is why a pH “drop of one point” is not a small movement. It is an order of magnitude, and it is why the difference between a tank at 7.6 and the same tank at 6.4 a fortnight later is a genuine emergency rather than a minor drift.

Goldfish themselves are unfussy. Anywhere from roughly 7.0 to 8.4 suits them, and they will live contentedly at either end; well-buffered pond water commonly swings between about 6.5 and 9 over a day without harm. What hurts is a large, fast change, or a fall below about 6.5, for the reason set out on the anatomy page: a fish maintains its internal chemistry against the water around it, and a sudden change in the water is work. A steadily falling pH is the warning sign.

So the target is not a number from a chart. It is whatever your tap water settles at, held reasonably steady.

The one that makes pH urgent: ammonia

Here is the interaction that makes pH worth understanding rather than merely recording.

Ammonia in water exists in two forms that interconvert constantly: ammonium (NH₄⁺), which is ionised and relatively harmless, and free ammonia (NH₃), which is un-ionised, crosses gill membranes readily, and is acutely toxic. The balance between them is set by pH and, to a lesser extent, temperature.

At low pH, almost all of it is ammonium. As pH rises, the free ammonia fraction climbs steeply.

Total ammonia reading	At pH 6.5	At pH 7.5	At pH 8.5
0.25 mg/L	Trace free ammonia; minor	Low but real	Meaningful; act
1.0 mg/L	Low but should not be there	Harmful over days	Dangerous today
2.0 mg/L	Harmful	Dangerous	Acutely dangerous

Most liquid test kits report total ammonia — both forms added together — and do not tell you the split. (Seachem’s badge and gas-exchange kits read free ammonia only, so this table does not apply to them.) That is why the same reading means different things in two different tanks, why an ammonia problem in hard alkaline water is more urgent than the number suggests, and why the water parameter checker asks for your pH before it interprets your ammonia.

There is a mirror-image version for nitrite, which is more toxic in soft water with low chloride — the mechanism described on the aquarium salt page.

KH: the thing that holds pH still

KH is carbonate hardness, also called alkalinity or buffering capacity, measured in degrees (dKH) or as mg/L of calcium carbonate. Roughly, 1 dKH is about 17.9 mg/L.

What it measures is the concentration of carbonates and bicarbonates dissolved in the water, and what those do is absorb acid. When acid enters the water, the carbonate reacts with it and is consumed; as long as some remains, the pH barely moves. When it runs out, the pH falls — and because the scale is logarithmic and there is nothing left to resist, it can fall a long way in a short time.

An aquarium produces acid continuously from two sources:

• Nitrification. The bacteria that convert ammonia to nitrite and nitrite to nitrate produce hydrogen ions as they work. This is a substantial ongoing acid load, and it scales with how much ammonia the tank produces — which, in a goldfish tank, is a great deal.
• Decomposition. Organic matter breaking down in the substrate, in the filter and in dead spots produces organic acids. The longer a tank goes between deep cleans, the more of this there is.

Water changes are what replenish KH, because fresh tap water brings its carbonate with it. So the whole system is a balance: acid production on one side, water changes on the other, and the KH reading tells you which is winning.

Around 4 dKH or above is comfortable. Below about 3, the buffer is thin enough that a busy week can exhaust it.

The pH crash

This is the practical payoff of the section above, and the failure mode behind a specific and common story: a tank that tested fine a few days ago, and now has sick or dead fish with no obvious cause.

The sequence runs like this. The tank is stocked heavily — a goldfish tank often is. The filter works hard, producing acid as it goes. Water changes are a little less frequent or a little smaller than the acid load requires. KH falls, slowly, over weeks, and nothing appears to happen, because that is exactly what a buffer is for. Then the KH reaches zero, the next day’s acid production has nothing to neutralise it, and the pH drops sharply — often a point or more, sometimes overnight.

Two things then happen at once. The fish experience a sudden, large chemistry change. And the nitrifying bacteria, which work poorly in acidic water, slow down or stall, so ammonia begins to accumulate in a tank whose pH is now also unstable. A pH crash tends to become an ammonia problem within days.

The signs are the ones a crash shares with everything else — clamped fins, gasping, lethargy — which is why the diagnosis comes from the test kit rather than the fish, and why testing KH occasionally is worth the small cost even though nothing appears to be wrong.

Old tank syndrome

A slower version of the same story, and one worth naming because it catches experienced keepers as often as new ones.

A tank runs for a year or two with water changes that are slightly too small or slightly too infrequent. Nitrate climbs steadily. KH is consumed and the pH drifts well below the tap value. Dissolved organics accumulate. None of it happens fast enough to notice, and the fish adapt, because fish are good at adapting slowly. The tank looks established and settled, and the fish look fine.

The danger is entirely in the correction. Someone tests the water, is alarmed, and does a 70 percent water change — replacing drifted, acidic, nitrate-heavy water with fresh tap water that is alkaline and clean. The fish, fully acclimated to the old conditions, meet a reversal of everything at once, and losses can follow.

The route out is the one in the callout above: 20 to 25 percent every day or two, for as long as it takes, testing as you go. The route to never needing it is regular water changes guided by a nitrate reading rather than by how the tank looks.

GH: mostly for everyone else

General hardness measures dissolved calcium and magnesium. It is often confused with KH and is a different thing entirely: a water can be high in one and low in the other, though in practice they usually move together because both come from the same rock.

Goldfish are tolerant of GH across a very wide range and, if anything, do better in harder water — not because they need the minerals particularly, but because hard water often brings carbonate buffering with it, and a well-buffered tank is a stable tank.

Where GH genuinely matters is the rest of the tank:

• Snails need calcium to build and maintain shells. In very soft water their shells pit and erode.
• Plants vary; many popular aquarium species prefer softer water, and a few of the ones tough enough for goldfish are indifferent.
• Very soft water is a problem for the aquarium as a system, because it usually has low KH and therefore pH instability. Water from a domestic softener is the exception, combining low hardness with high KH, so test KH rather than inferring it from GH.

If your water is hard, that is a convenience for goldfish keeping. Do not correct it.

Reading your own tap water

Do this once, properly, and it will explain most of what your tank does afterwards.

Test straight from the tap, and then test a glass of the same water left standing for 24 hours. The two readings often differ, and the reason is dissolved carbon dioxide: mains water is frequently supersaturated with CO₂, which forms carbonic acid and depresses the pH. Standing lets the CO₂ come out, the carbonic acid goes with it, and the pH rises — sometimes by half a point or more.

The settled reading is the one your tank will actually sit at. Testing only fresh tap water is how people conclude their water is more acidic than it is, and occasionally how they end up adding buffer they did not need. Tap water and source water covers where your hardness comes from, and the softener, well or RO supply that changes the answer entirely.

While you are there, test KH and nitrate too. A tap KH below 3 to 4 dKH tells you that buffering is a permanent consideration rather than an occasional one. Tap nitrate of 20 mg/L or more, which is not unusual in farming regions, sets a floor beneath which no water change can take you.

Units, and why two people compare different numbers

A small practical trap. KH and GH are reported in at least three ways, and test kits from different countries pick different ones.

Unit	Also written	Conversion
German degrees	dKH, dGH, °dH	1 degree ≈ 17.9 mg/L as CaCO₃
Parts per million	ppm, mg/L as CaCO₃	the direct measure
Milliequivalents	meq/L	1 meq/L ≈ 50 mg/L ≈ 2.8 degrees

So “my KH is 5” means 5 German degrees to a European keeper and may mean 5 ppm to someone reading a strip calibrated in mg/L — which is the difference between a comfortably buffered tank and one about to crash. When comparing notes, or reading advice online, check which unit is in play; the glossary has the other units a keeper meets, from ppm to the two sizes of gallon. A liquid kit that counts drops is usually reporting German degrees, where each drop is one degree; a strip usually prints mg/L.

pH has no such problem, but it has a subtler one: strips are poor at it. A strip will tell you roughly where you are, and for a reading whose whole value lies in detecting small movements over time, roughly is not much use. This is one of the places where a liquid test kit earns its cost.

The daily swing in a planted tank

If your tank has meaningful plant growth, the pH is not one number — it is a cycle.

Plants consume carbon dioxide in the light and release it in the dark. Carbon dioxide dissolved in water forms carbonic acid, so as the plants strip it out during the day the water becomes less acidic and the pH rises; overnight, as respiration puts CO₂ back, the pH falls again. In a heavily planted tank with low KH the daily swing can be several tenths of a point or more.

Three practical consequences:

• Test at the same time of day. Comparing a morning reading with an evening one is comparing two different points in a cycle, not measuring a change in the tank.
• KH damps the swing. This is one more argument for adequate buffering, since the same plant activity moves a well-buffered tank far less.
• Goldfish tanks rarely have this problem, because goldfish eat most of the plants. Where it appears, it is usually a tank with tough species tied to wood or rock and strong lighting.

Chemistry while a tank is cycling

The interaction between pH and the nitrogen cycle is most acute in a new tank, and it catches people out at the worst possible moment.

Nitrifying bacteria work best in neutral to alkaline water and slow markedly as it becomes acidic, largely stalling below about pH 6. At the same time, their own activity produces acid and consumes KH. In a soft-water tank, a cycle can therefore stall itself: the bacteria acidify the water until they can no longer function, the ammonia stops being processed, and the readings sit stubbornly still for weeks while the keeper concludes the cycle has failed.

The diagnosis is straightforward if you test for it. A cycle that has stopped progressing, with ammonia or nitrite refusing to fall, in water whose pH has drifted downward and whose KH is at or near zero, is not a bacterial problem — it is a buffering one. Restore the KH and the cycle resumes.

The cycling guide covers the process itself. The chemistry note to add is simply this: if you are cycling in soft water, test KH as well as the nitrogen readings, and keep it above about 4 dKH throughout.

A worked example

Numbers in isolation are hard to interpret, so here is a whole tank read together.

A 150-litre tank, two fancy goldfish, running for eight months. Water changes of about 25 percent, most weeks. Readings, taken in the evening:

• Ammonia 0 mg/L
• Nitrite 0 mg/L
• Nitrate 40 mg/L
• pH 7.0, where the settled tap water reads 7.8
• KH 2 dKH, where the tap reads 6

Individually, four of those five look acceptable. Ammonia and nitrite are zero, the filter is clearly working. Nitrate at 40 is at the top of the range but not alarming. pH 7.0 is squarely in the goldfish comfort band.

Read together, they describe a tank heading for trouble. The nitrate says water changes have been slightly too small or too infrequent for the bioload. The KH, a third of the tap value, says the buffer has been consumed faster than it is being replaced — and at 2 dKH there is very little left. The pH, 0.8 below the tap value, confirms it has already started to fall. This tank is perhaps a fortnight from a crash, and the crash will take the filter with it.

The fix is not a pH product. It is: increase the water changes to 40 percent weekly or 25 percent twice weekly to bring nitrate down and carbonate back up, vacuum the substrate properly to remove the organic load producing acid, and — because the tap KH of 6 is on the low side for a tank this heavily stocked — put a bag of crushed coral in the filter as a permanent floor.

Three weeks later the same tank should read nitrate around 20, KH near 5, and pH within a couple of tenths of the tap value. Nothing was dosed.

Diagnosing from the combination

The readings are more informative together than separately.

pH	KH	What it usually means
Stable, matches settled tap	4 dKH or above	Healthy; nothing to do
Stable but well below tap	Low	Drifted over time — old tank syndrome; correct slowly
Falling week to week	Falling	Buffer being consumed faster than it is replaced
Falling	Already near zero	A crash is imminent or has begun
Higher than tap	Normal or high	Usually decor — limestone, coral, shells dissolving
Swings after water changes	Low	Not enough buffer to absorb the difference

The row worth dwelling on is the fifth. A pH persistently above the tap value, in a tank nobody has dosed, often means something in the tank is dissolving: a limestone rock, a coral or shell ornament, or a substrate that was sold as inert and is not. The decorations guide covers the vinegar test, which takes ten seconds and answers it.

Adjusting, and the strong case for not

Most goldfish tanks need no chemistry management at all. The ones that do often need buffering rather than pH adjustment, and the distinction is the whole point.

To raise KH, slowly and safely: crushed coral or aragonite in a media bag in the filter. It is an elegant solution because it is self-regulating — the material dissolves faster as the water becomes more acidic and slower as it becomes alkaline, so it tends to hold a floor rather than pushing the pH up indefinitely. Add a small amount, test after a week, and adjust.

To raise KH quickly, in an emergency: sodium bicarbonate — ordinary baking soda — will do it, and it is the right tool for a crashing tank. It is also fast and easy to overdo, so it is emergency medicine rather than maintenance.

To reduce hardness, which is rarely necessary for goldfish: blending in reverse-osmosis water. RO water has essentially no buffer at all, so it must be blended rather than used neat, or you create the instability you were trying to avoid.

What not to use: the bottled pH adjusters. pH-down products use up KH and pH-up products add it, so the number moves, then drifts back as the tap water, the decor and the carbon dioxide reassert themselves, leaving less buffer than before, and the fish get a swing in each direction for nothing. A tank that needs weekly pH adjustment has a buffering problem that the adjuster is hiding.

When hardness is genuinely a problem

Rarely, and when it is, the problem is nearly always softness rather than hardness.

Genuinely soft tap water — under about 3 dKH — makes a goldfish tank harder to run, because a heavy bioload plus minimal buffering is exactly the combination that crashes. If that describes your supply, the answer is not to fight it but to plan around it: crushed coral in the filter as a permanent fixture, larger and more frequent water changes, a lighter stocking level, and a KH test used regularly rather than once a year.

Very hard water is not a goldfish problem. It limits your plant choices and it leaves scale on the glass, and that is the extent of it.

The short version

Test your settled tap water once so you know your baseline. Keep KH at 4 dKH or above and the pH will look after itself. Do not chase a pH number with a bottle; if the pH is moving, find what is consuming the buffer. Remember that the same ammonia reading is more dangerous in alkaline water. And when any chemistry has drifted a long way, bring it back more slowly than it left.