Every tank I have ever kept has taught me the same lesson eventually: the water is the pet. The fish are just the visible part of a system that lives or dies on chemistry you cannot see. When something goes wrong in an aquarium, the answer is almost never in the fish themselves. It is in the numbers. The good news is that there are only a handful of numbers that matter, and once you understand what each one is actually telling you, testing stops being a chore and starts being a conversation with your tank. Here is what each parameter means, what a healthy reading looks like, and what to do when a reading is off.

Ammonia: the one that must always be zero

Ammonia is what fish excrete, mostly through their gills, and it is also what uneaten food and dead plant matter release as they rot. It is genuinely toxic. Even low levels burn gill tissue, stress the immune system and open the door to disease. At higher levels it kills outright, and it kills quickly.

In a healthy, cycled tank, ammonia should read zero. Not “low”. Zero. The bacteria living in your filter media and on every surface in the tank consume ammonia as fast as the fish produce it, so a working tank never shows a reading at all.

Any measurable ammonia means something is wrong, and your job is to work out what. The usual suspects, roughly in order of likelihood:

  • The tank was never cycled. This is the classic new-tank disaster. If you added fish to a tank that has not been through cycling, there simply are not enough bacteria yet to process the waste. Read that guide, because it explains the whole process properly.
  • Something died. A missing fish or snail decomposing behind the hardscape can spike ammonia in a day or two. Count your fish. Poke around.
  • Overstocking or overfeeding. More waste than the bacteria can handle. Feed less, and be honest with yourself about how many fish the tank can carry.
  • A filter crash. Bacteria die if the filter stops running for hours, if you clean the media in chlorinated tap water, or if a medication wipes them out. Always rinse filter media in old tank water, never under the tap.

Whatever the cause, the immediate response is the same: water changes. A 50 percent change halves the ammonia concentration. Do one straight away, test again, and repeat daily until you read zero. Water changes buy your fish time while you fix the underlying problem. They are not the fix themselves, so keep testing until you know why the reading appeared.

Nitrite: the second stage, just as dangerous

Nitrite is what the first group of bacteria turn ammonia into. It is roughly as dangerous as ammonia, just in a different way. Nitrite binds to haemoglobin in the fish’s blood and stops it carrying oxygen, so fish in nitrite-laden water are slowly suffocating even in well-aerated water. You will sometimes see them gasping at the surface or sitting near the filter outflow.

Like ammonia, nitrite should always read zero in an established tank. The second group of bacteria consumes it as fast as the first group produces it.

Where you will see nitrite is during cycling. A cycling tank shows an ammonia spike first, then a nitrite spike as the first bacterial colony gets going before the second one has caught up. This is normal and expected in a fishless cycle. It is the reason fishless cycling exists, because putting fish through that nitrite spike harms them.

If nitrite shows up in an established tank, treat it exactly like ammonia: water change immediately, then find the cause. The causes are the same list as above, since anything that crashes your ammonia-eating bacteria tends to take the nitrite-eaters down with it. One extra trick worth knowing: a small amount of aquarium salt, around one gram per litre, blocks nitrite uptake at the gills and gives fish real protection while you sort the tank out. Check your species tolerate salt first, but most community fish handle that level fine for a short period.

Nitrate: the end of the line

Nitrate is what nitrite becomes, and it is the end product of the nitrogen cycle in most home aquariums. It is far less toxic than the other two. Fish tolerate levels that would be catastrophic if they were ammonia. That does not make it harmless. Chronically high nitrate stresses fish, stunts fry, shortens lives and feeds algae. It just does its damage over months rather than hours.

I aim to keep nitrate under 20 ppm, and I would call anything under 40 ppm acceptable for hardy community fish. Sensitive species and anything you hope to breed will do better at the low end.

Two tools keep nitrate down. The first is the humble water change, which physically removes it. If your nitrate creeps up week on week, your water change schedule is not keeping pace with your stocking, and either the changes get bigger or the feeding gets smaller. If you suspect the tank is simply carrying too many fish, the aquarium stocking calculator gives a quick read on how heavily stocked you are. The second is live plants, which consume nitrate as fertiliser. A heavily planted tank can run remarkably low nitrate with modest maintenance, and fast growers like hornwort, water sprite and floating plants are the greediest of the lot.

One thing that catches people out: test your tap water. Many supplies, particularly in agricultural areas, come out of the tap already carrying 10, 15, even 20 ppm of nitrate. Mine does. If your tap water starts at 16 ppm, a water change can never get the tank below that, and your realistic target shifts accordingly. You cannot know this without testing the tap, which takes two minutes and settles the question permanently.

pH: stability beats perfection

pH measures how acidic or alkaline your water is, on a scale where 7 is neutral, lower is acidic and higher is alkaline. Ask a beginner what pH their fish need and they will quote a number from a care sheet. Ask anyone who has kept fish for a decade and they will tell you the number matters far less than whether it holds still.

Most aquarium fish are farmed, many generations removed from the wild, and they adapt comfortably to a stable pH anywhere from the low 6s to around 8. What they do not adapt to is a pH that lurches around. A swing of a full point in a day is a serious physiological shock. A steady 7.8 is vastly better for a “soft water” tetra than a tank that bounces between 6.5 and 7.5 because someone keeps dosing it.

This is why I tell people never to chase pH with bottled chemicals. The pH-up and pH-down products on the shop shelf produce exactly those swings. They shift the number for a day, the water’s natural buffering drags it back, you dose again, and your fish ride a chemical seesaw. I have seen more fish killed by pH adjusters than by the “wrong” pH.

The sane approach runs the other way: test your tap water, accept what it is, and choose fish that suit it. Hard alkaline water? Livebearers, rainbowfish and Rift Lake cichlids will thrive. Soft acidic water? Tetras, rasboras and many dwarf cichlids are yours. If your pH is genuinely stable and somewhere between 6.5 and 8, almost everything commonly sold will live happily in it. For the short answer on a mixed tank, see what pH a community tank should be.

GH: what your fish actually feel

General hardness, or GH, measures the dissolved calcium and magnesium in your water. This is the parameter fish physiology genuinely cares about, more than pH in most cases. Fish constantly regulate the balance of water and minerals across their gills and skin, a process called osmoregulation, and the mineral content of the water sets how hard they have to work at it. Fish also use calcium and magnesium directly, for bones, scales and eggs.

GH is usually given in degrees (dGH) or ppm. Roughly, 0 to 4 dGH is soft, 4 to 8 is moderately soft, 8 to 12 is moderately hard, and above that you are into properly hard water. My own tap runs over 20 dGH, which rules out sensitive soft-water species but is a gift for livebearers and shell dwellers.

Like pH, GH is something to work with rather than against. Soft-water fish kept in liquid rock struggle to breed and often live shortened lives; hard-water fish kept in very soft water suffer mineral deficiencies. If your water is hard, there is a genuinely long list of fish that will not just cope but flourish, and I have written up my favourites in the hard water guide. If your water is very soft, perhaps from a rain butt or an RO unit, you may actually need to add minerals back in rather than take them out.

KH: the buffer that keeps pH honest

Carbonate hardness, or KH, measures the carbonates and bicarbonates dissolved in your water. It sounds like a sibling of GH, and test kits usually bundle them together, but it does a completely different job. KH is your tank’s buffering capacity. It is the chemical shock absorber that soaks up the acids a tank constantly produces and holds the pH steady.

This makes KH the hidden explanation behind most pH mysteries. A tank with a decent KH, say 4 dKH or more, holds its pH almost regardless of what you do to it. A tank with very low KH has nothing holding the pH up, and as fish waste and decaying matter generate acid, the pH drifts down and can eventually crash, sometimes dropping a full point or more overnight. Old, neglected tanks with infrequent water changes are the classic victims: the buffer gets consumed over months, nothing replaces it, and one day the pH falls off a cliff and the fish are gasping.

So test KH occasionally even if your pH looks fine. If your tap water carries good KH, routine water changes top up the buffer and you will likely never have a problem. If your source water is very soft, or you use RO water, you need to add buffering back deliberately. Working out how much mineral salt to dose is fiddly by hand, which is why I built a remineralisation calculator that does the arithmetic for you for both GH and KH targets.

Temperature: brief, because the rule is simple

Temperature barely needs its own section, but it is a water parameter and the same logic applies: stable and species-appropriate. Most tropical community fish want somewhere between 23 and 26°C, though some species sit well outside that band, so check yours specifically. A cheap thermometer that you actually look at beats trusting the dial on the heater, because heaters fail, and they fail in both directions. What harms fish most is not being a degree off the care sheet but daily swings, so keep tanks away from radiators, direct sun and draughty windowsills. If you are still choosing a heater, here is how to work out what size heater you need in watts per litre.

How to test, and how often

Get a liquid reagent test kit. The paper dip strips are tempting because they are cheap and fast, but they are vague at best and wildly wrong at worst, especially for ammonia, and ammonia is the reading where wrong answers cost lives. A liquid master kit covering ammonia, nitrite, nitrate and pH costs about the same as two or three fish and lasts for hundreds of tests. Add a GH/KH kit, which is usually sold separately.

Test your tap water first, before anything else. Tap water is the baseline every water change pulls your tank towards, and until you know its pH, GH, KH and nitrate, you are guessing about everything downstream.

For routine, my honest schedule looks like this. During cycling or after any problem, test ammonia and nitrite daily. In an established, stable tank, a nitrate check every week or two tells you whether your maintenance is keeping up, and a full run of everything once a month or when anything looks off. Fish behaving oddly, cloudy water, an unexplained death: test before you treat, every time. Medicating a tank without knowing the water chemistry is treating symptoms blind.

Do not let any of this tip into obsession. I know keepers who test everything daily and change nothing but their own stress levels. The point of knowing your numbers is confidence, not anxiety. A cycled, sensibly stocked, regularly maintained tank will show the same boring readings month after month, and boring readings are exactly what you want. Learn what your tank’s normal looks like, and then testing becomes what it should be: a quick check that everything is still quietly working, and an early warning on the rare day it is not.