How to adjust your aquarium water parameters

Not a parameter you adjust. Any detectable reading is an emergency, because ammonia damages gills and kills quickly, and the only permanent answer is a working filter.

1. Water change now, generously, with temperature-matched dechlorinated water. This is the fastest way to reduce the concentration.
2. Use a conditioner that binds ammonia, such as Seachem Prime, which detoxifies it temporarily while the filter catches up.
3. Stop feeding for a day or two, since food is the source.
4. Find the cause. A new tank that was never cycled, a filter that was over-cleaned or has stopped, a dead fish, or a sudden increase in stocking. See how to cycle a new aquarium.

The same reading is not equally dangerous everywhere. Ammonia sits in a balance between two forms, and the genuinely toxic one becomes far more prevalent as pH and temperature rise. So 0.5 ppm in alkaline Brisbane water on a hot afternoon is considerably more serious than the same reading in soft, near-neutral Melbourne water overnight. Do not take comfort from a low number if your pH is high.

Never try to fix it with chemicals alone while ignoring the cause. Everything above buys time; the filter is what actually removes ammonia.

The second stage of the nitrogen cycle, and equally an emergency rather than an adjustment. Nitrite usually appears a week or two into a cycle, or when a filter has been damaged and the second group of bacteria has not kept up.

It harms fish differently from ammonia. Nitrite binds to the blood so oxygen can no longer be carried, which means a fish can suffocate in water that is perfectly well oxygenated. Affected fish gasp at the surface, which looks like an oxygen problem and is not one, so aeration alone will not save them.

Salt gives real protection. Chloride competes with nitrite for uptake at the gills, so a modest dose of aquarium salt substantially reduces the harm while you deal with the underlying cause. This is worth knowing because it buys time that nothing else will. Check first that your stock tolerates salt, since some catfish and loaches are sensitive.

Otherwise the response is the same as for ammonia: water change now, a conditioner that binds it, stop feeding, and find the cause. See the ammonia card above and our cycling guide.

The fastest-moving parameter here, and the one where a sudden change does the most immediate harm.

To raise: a correctly sized heater, ideally with a separate thermometer on the far side of the tank so you are not trusting the dial. Size it with our heater wattage calculator.

To lower: the full method is in how to cool an aquarium in an Australian summer, but in short: surface agitation and an airstone first because the real danger of heat is oxygen loss, then lights off, then a fan across the surface or a sealed frozen bottle, then cool the room.

Either direction, go gradually. Around 1°C per hour is a sensible ceiling. Match water-change water to tank temperature rather than correcting afterwards.

GH is the dissolved calcium and magnesium. It matters most to fish for osmotic balance, and to shrimp and snails for moulting and shell building.

To raise GH: a dedicated remineraliser such as Seachem Equilibrium raises GH without touching KH, which is the precise tool when you want minerals but not a pH change. Crushed coral also raises GH, but it raises KH and pH along with it. For shrimp tanks, a purpose-made shrimp mineral gives the tightest control.

To lower GH: realistically, dilution is the only practical method. Mix RO water or collected rainwater with your tap water in a measured ratio, and keep that ratio identical at every water change. There is no reliable chemical that removes hardness from a running tank.

Worth knowing: GH is the parameter most often quoted wrongly, because water utilities use a different hardness scale from aquarists. See the water parameters guide for the aquarium scale.

KH is your tank's shock absorber. It neutralises the acids a tank produces continuously, and it is what stops pH moving. Low KH is the single most common cause of pH instability.

To raise KH: crushed coral or aragonite in the filter or substrate is the standard answer, because it dissolves only as the water becomes acidic and therefore self-regulates. A commercial carbonate buffer gives faster, more precise control. Sodium bicarbonate is also used, but it raises KH without raising GH, so it changes the balance between them; add it dissolved in a bucket, in small amounts, testing as you go.

To lower KH: dilute with RO or rainwater, or filter through peat. This is mostly done for soft-water breeding projects and for Caridina shrimp, where high KH prevents the low pH they want.

A useful floor: around 3 to 4 dKH is enough to hold pH steady in most community tanks. Below about 2 dKH, watch pH closely, especially if the tank is heavily stocked or injecting CO2.

Understand this first: pH is not really an input you set. It is the result of your KH (which resists downward movement) and your dissolved CO2 (which pushes it down). Adjust those and pH follows. Attack pH directly and you are fighting the buffering, which is why bottled adjusters give a drop, a rebound, and a stressed tank.

To raise pH: raise KH, using crushed coral or aragonite in the filter or substrate. It dissolves slowly, is self-limiting, and lifts pH gently and permanently. Increasing surface agitation also helps, by driving off dissolved CO2, which is often all a tank with a slightly low pH needs.

To lower pH: dilute with RO or rainwater to reduce the KH holding it up. Botanicals such as Indian almond leaves, alder cones, peat and driftwood release tannins and organic acids that gently lower it, and they suit blackwater setups. Injected CO2 lowers pH directly and predictably, which is why planted tanks run lower (see our CO2 guide).

If your pH is falling on its own, the cause is exhausted KH, and the answer is more frequent water changes plus some carbonate buffering, not a pH product. This is common in very soft cities such as Melbourne.

Nitrate is the end product of the nitrogen cycle, far less toxic than ammonia or nitrite, but it accumulates and a persistently high reading tells you something about how the tank is run.

To lower it: water changes are the direct method, and consistent moderate changes work better than occasional large ones. Underneath that, treat the causes: feed less, stock less heavily, vacuum the substrate properly, and grow fast-growing plants, which consume nitrate as fertiliser. Nitrate-removing resins exist and work, but they treat the symptom.

Check your tap first. Some supplies carry meaningful nitrate, and bore water in particular can be very high. If your tap already reads high, water changes cannot take the tank below it (see bore water). Most city supplies are effectively nitrate-free, so a rising reading is coming from inside the tank.

To raise it: only relevant in heavily planted tanks where plants have stripped the water bare, in which case you are dosing fertiliser rather than adjusting water.