Fish Tank Water Salinity and Hardness

By Sophie Kininmonth · 8 Oct 2025 (updated 21 Jul 2026) ·5 min read

Stable water chemistry is the single biggest thing standing between a healthy tank and a struggling one, and two parameters do most of the quiet work: salinity and hardness. Get them right and the fish barely notice; let them drift and you see faster breathing, refused food, and creeping disease long before the water ever looks dirty. The good news is that these are two separate, measurable levers, and once you know which one applies to your tank the fixes are simple. Salinity is about dissolved salts overall, mostly a saltwater question. Hardness is about calcium and magnesium, mostly a freshwater question.

Salinity: what it is and when it matters

Salinity is the total amount of dissolved salts in water, and in a marine tank it is the backbone of everything else. It affects osmoregulation, oxygen exchange, and how other parameters behave, including alkalinity and calcium balance. In freshwater tanks salinity is usually close to zero, unless you deliberately add salt or keep brackish species. Natural seawater sits around 35 ppt (often written as 35 PSU in scientific contexts).^{1, 2}

Recommended salinity targets

Freshwater tanks: generally aim for near 0 ppt unless a specific species or treatment plan calls for salt. Adding salt "just because" often causes more harm than good.
Marine (reef or fish-only): aim for around 35 ppt, commonly read as roughly 1.026 specific gravity at 25 °C, depending on the instrument.^{1, 3}

Specific gravity versus ppt

A common trap is treating specific gravity (SG) as a universal number. SG readings depend on temperature and on how the hydrometer or refractometer is calibrated, so the same true salinity can produce different SG values on different tools.³ If you keep a marine tank, it is usually clearer to track salinity as ppt (or conductivity) and to calibrate your refractometer properly.

How to measure salinity accurately

Hydrometer (budget-friendly, but variable)

Plastic swing-arm hydrometers are common and cheap, but they are easy to misread and sensitive to bubbles, residue, and temperature effects. They can be fine for rough checks, though they are not ideal for tight marine stability.

Refractometer (preferred for marine tanks)

A refractometer is usually the best day-to-day tool for marine aquariums, as long as you calibrate it with a 35 ppt seawater calibration solution rather than pure water. Many measurement errors come from incorrect calibration and temperature assumptions.³

Common salinity problems

Salinity creeping higher

In marine tanks, rising salinity is almost always evaporation. Water leaves, salt stays.

Quick check: has the waterline dropped between top-ups?
Safer fix: top up with fresh (RO/DI) water, not saltwater, then re-test.

Salinity dropping lower

Low salinity in a marine tank is usually caused by topping up evaporation with saltwater (compounding the mistake), overly large freshwater additions, or inaccurate readings.

Safer fix: raise salinity slowly by replacing small amounts with correctly mixed saltwater, checking between adjustments.

Water hardness: what it really measures

Hardness is mostly a freshwater topic, and it comes in two forms that are often mixed up:

GH (general hardness): a measure of dissolved divalent minerals, mainly calcium (Ca²⁺) and magnesium (Mg²⁺). It is commonly reported as °dGH or as mg/L (ppm) CaCO₃ equivalents.^{4, 5}
KH (carbonate hardness or alkalinity): a measure of carbonate and bicarbonate buffering, which helps resist rapid pH change.⁶

They are related but not the same thing. You can have high KH with modest GH, or high GH with modest KH, depending on what is dissolved in the water.⁷

Units: °dGH, °dKH and ppm

Hardness kits may report results in "German degrees". As a working conversion, 1 °dH is roughly 17.8 mg/L (ppm) as CaCO₃.^{4, 8}

Does hardness increase salinity?

Only slightly, in most freshwater contexts. Calcium and magnesium are dissolved ions, so they do contribute to total dissolved solids, but GH and KH are usually discussed separately from salinity because they influence fish and pH stability in different ways, and at very different concentration scales in typical aquariums.⁷

How to test GH and KH

Most aquarium GH/KH tests are simple titration kits: you add drops until the colour changes, then count the drops to get °dGH or °dKH. Test:

your tap water, so you know what you are starting with,
your tank water, to see what has shifted,
and after any adjustments, to confirm you moved the number you meant to move.

Managing hard water

If GH/KH are too high for your fish

Dilute with RO/DI water (or demineralised water) during water changes, then re-mineralise only as needed for your species.
Avoid "pH-down" quick fixes when KH is high. The buffer will push back, and the tank can become unstable.

If GH/KH are too low

Raise KH with controlled carbonate or bicarbonate sources (used carefully), or with a remineraliser designed for aquariums.
Raise GH with calcium and magnesium remineralisers rather than guessing with mixed products.

In practice, stability matters more than chasing a single perfect number. Many fish do well across a range but struggle when conditions swing.

Simple habits that keep chemistry steady

Top up evaporation correctly: marine tanks lose water, not salt, so top up with fresh water.
Mix saltwater consistently: same salinity target, same temperature window, and allow time for full dissolution before use.
Test before you correct: one accurate reading saves a week of undoing guesswork.
Make changes slowly: fish can often adapt to a stable "not-perfect" parameter better than to a rapid adjustment.

The Bottom Line

Salinity is the headline parameter for marine tanks, and hardness (GH and KH) is often the quiet driver behind freshwater stability. Keep the two separate in your mind, measure with tools you trust, and adjust in small steps. A settled tank is a healthy one, and the fish settle with it.

References

Sophie Kininmonth

Dog lover and writer at My Pets Australia, sharing practical breed guides and care advice for Australian pet owners.