Do mollies need salt in their tank?

No. Mollies need hard, mineral-rich alkaline water — GH 12–25, pH 7.5–8.5 — and salt is a crude substitute that treats the symptom of soft water rather than the cause.

Why do people say mollies need salt?

Because mollies are euryhaline and genuinely tolerate brackish water, and because adding salt to soft water raises osmotic pressure and makes struggling fish improve. They needed minerals, not salt.

Is aquarium salt bad for mollies?

Not for mollies — they tolerate it better than almost any freshwater fish, which makes salt a legitimate treatment for them. It harms scaleless tank mates and plants, so it is a poor permanent additive.

How do I make water harder for mollies?

Crushed coral or aragonite in the filter or as substrate, a GH/KH remineraliser if you use RO, or limestone hardscape. Raise it over a few weeks rather than in one change.

Do mollies need salt?

No — but the advice is not as wrong as the correction usually claims.

This question generates more bad advice than anything else in livebearer keeping, in both directions. One camp insists mollies are brackish fish that need salt to survive. The other insists salt is unnecessary and harmful to plants and tank mates. Both are working from a partial picture.

The resolution: mollies need hard, mineral-rich, alkaline water. Salt is one crude way of making water feel more like that, and it treats the symptom of soft water rather than the cause.

Common nameMolly

Water TypeFreshwater — hard and alkaline; salt tolerant

Key checksGH 12–25 dGH · KH 10–20 dKH · pH 7.5–8.5

ApproachFix hardness, not salinity. Salt is a treatment, not a requirement.

Where the myth comes from

It comes from a true fact. Poecilia mollies are genuinely euryhaline — they tolerate an exceptionally wide salinity range, and wild populations occur in fresh water, brackish estuaries and even full marine conditions. Sailfin mollies in particular are found in coastal salt marshes.

So mollies can live in brackish water. That is not the same as needing it, and the leap from one to the other is where the advice goes wrong.

What is actually going on is this. Mollies come from hard, alkaline, mineral-rich water with high dissolved solids. When a keeper puts them in soft tap water they do badly — faded colour, clamped fins, shimmying, disease. Adding salt raises dissolved solids and osmotic pressure, the fish improve, and the keeper concludes mollies need salt.

They did not need salt. They needed minerals, and salt was a blunt instrument that partially delivered them.

What they actually need

GH 12–25 dGH. KH 10–20 dKH. pH 7.5–8.5.

That is harder and more alkaline than almost any other common community fish wants, and harder even than guppies require — mollies are the most demanding of the common livebearers on this axis.

If your tap water delivers that, you need nothing. No salt, no additives.

If your water is soft, fix the hardness properly:

Crushed coral or aragonite in the filter or as substrate. Dissolves slowly, raises GH and KH, and self-limits. The standard answer.
A GH/KH remineraliser if you run RO water.
Limestone or texas holey rock as hardscape.

All of these raise the minerals the fish are short of, without the side effects of salt.

When salt genuinely helps

Salt is a legitimate tool for mollies in specific circumstances — this is why the old advice will not die.

As a disease treatment. Aquarium salt at 1–3 g/L is effective against some external parasites and helps with osmotic stress in a sick fish. Mollies tolerate this better than most fish, which makes salt a reasonable treatment option for them where it would be risky for others.

As a temporary support for a fish shimmying in soft water while you sort out the hardness properly. It buys time. It is not the fix.

In a dedicated brackish tank, where mollies are an excellent choice alongside other brackish species. This is a deliberate setup, not a default.

What salt does not do is provide general "tonic" benefit. Permanent low-dose salt in a freshwater community tank is not maintenance — it is a constant mild osmotic load on every fish that is not a molly.

Why permanent salt is a bad default

It harms most tank mates. Corydoras, kuhli loaches, most tetras and nearly all catfish are scaleless or salt-intolerant. A molly community dosed with salt is a bad tank for everything except the mollies.

It kills most plants. Salt at meaningful concentrations damages or kills common aquarium plants, which removes the cover and nitrate uptake a livebearer tank benefits from.

It does not evaporate. Salt stays when water leaves, so topping up evaporation with fresh water and salted water alternately produces a slowly drifting salinity nobody is tracking.

It masks the real problem. A tank that needs salt to keep mollies healthy is a tank with the wrong hardness, and that will keep causing problems.

The practical decision

Test your GH and KH. This is the whole question, and one test kit answers it.
GH 12+ and pH above 7.5? Keep mollies, add nothing.
Softer than that? Add crushed coral and raise hardness over a few weeks. Do not add salt.
Fish shimmying right now? Salt at 1–2 g/L as short-term support while the hardness comes up — see molly shimmies and health.
Want a brackish tank? Mollies are one of the best candidates, at SG 1.005–1.015, with tank mates chosen to match.