Water Testing: What to Test and When

Most aquarium problems are invisible until something is measured. Ammonia and nitrite are toxic far below any concentration a person can detect, pH moves during the day, and nitrate climbs silently. A test kit converts all of that into numbers you can act on.

The five parameters that matter

Water Testing: What to Test and When — data
Parameter	Target	Test when	Action if out
Ammonia	0 ppm	Daily during cycling	Water change, find the source
Nitrite	0 ppm	Daily during cycling	Water change, do not stock
Nitrate	10–20 ppm	Weekly, before water change	Larger or more frequent changes
pH	Stable, 6.5–7.5 typical	Weekly, same time of day	Check KH; do not dose adjusters
GH / KH	GH 6–10, KH 3+ dKH	Monthly, weekly when adjusting	Remineralise gradually

Ammonia and nitrite: the emergency pair

These two are the only readings that require immediate action. Both are toxic at concentrations below 1 ppm, and neither is visible. During the initial cycle they should be tested daily; in an established tank, test them whenever something changes — a new fish, a dead fish you have removed, a filter that has been off, a medication course.

A reading above zero in an established tank is a cycle problemAmmonia or nitrite appearing in a tank that was previously reading zero means the bacterial colony has been damaged — usually by chlorine, by washing media under a tap, or by a filter left unpowered. Water change daily and stop feeding until it reads zero twice in a row.

Nitrate: the number that sets your routine

Nitrate is the end of the nitrogen cycle and nothing in a normal filter removes it. Two things export it: water changes and fast-growing plants. In a heavily planted, lightly stocked tank, plants can consume enough that nitrate reads near zero, which is fine.

Test before the water change, not after. The pre-change number is the one that tells you whether your schedule is keeping up. A reading that climbs across three consecutive weeks means bioload has outgrown the routine, and the fix is either a larger change, a more frequent one, or fewer fish.

Shake the bottle for the full thirty secondsMost liquid nitrate kits reduce nitrate to nitrite before colour-matching, and the reducing agent settles. Skipping the shake under-reads by a factor of two or more. A kit that always reads zero in a heavily stocked tank is a kit you should suspect before you congratulate yourself.

pH: stability beats the number

pH is a logarithmic scale, so one unit is a tenfold change in hydrogen ion concentration. But the value itself matters far less than its stability. A tank holding steady at 7.4 is a better environment for most fish than one oscillating between 6.6 and 7.2.

In a CO₂-injected tank, pH falls through the photoperiod as dissolved CO₂ rises and recovers overnight. A drop of about 1.0 unit between lights-on and lights-off is normal and is itself a rough confirmation that CO₂ is at target. A drop of more than 1.5 units, or a value that will not settle, indicates a problem — most often depleted KH.

Test at the same time of day, every time. A morning reading and an evening reading of the same tank are legitimately different numbers.
Read the tube against a white card in daylight. Coloured backgrounds and warm bulbs distort the match.
Never use pH up or pH down products. They cause swings that kill livestock. pH is set by substrate, water source and CO₂.
If pH will not hold steady, test KH. KH is the buffer, and low KH means pH is at the mercy of every input.

GH and KH: the numbers that decide which species you can keep

These two are routinely confused. GH — general hardness — is dissolved calcium and magnesium. It determines whether shrimp can moult and whether plants can take up calcium. KH — carbonate hardness — is carbonate and bicarbonate, and it determines how resistant pH is to change.

Both are measured by counting drops until a colour change. GH is typically red to green; KH is blue to yellow. Counting drops is accurate enough, provided the tube is shaken between drops.

The aqua soil trapAqua soil strips KH. A tank that starts at 5 dKH can reach 0 within weeks, at which point pH becomes unstable and the tank is one water change away from a crash. Test KH weekly for the first two months of any aqua soil setup, and add potassium bicarbonate if it reaches zero.

The ten-minute weekly routine

Nitrate, before the water change

One test, five minutes including the shake. This is the number that tells you whether the routine is working.

pH, at the same time as last week

Consistency of timing is what makes the series meaningful.

Ammonia and nitrite, if anything has changed

New fish, a death, a filter outage, a medication course. Otherwise monthly is enough in a stable tank.

GH and KH, monthly

Or weekly if you are adjusting them, running aqua soil, or injecting CO₂.

Write it down

A single reading tells you the current state. A series tells you the direction of travel, which is the more useful information and the thing you cannot reconstruct from memory.

Ten minutes a week. It is the cheapest insurance in the hobby, and it is the difference between finding a problem while it is small and finding it after the fish have died.