Molly Fish Temperature & Water Chemistry, Explained

Published: 2026-09-17 · FishyKart Blog

Molly fish need 24-28°C (75-82°F) water with a pH of 7.5-8.5 and 15-30 dGH hardness — harder and more alkaline than most freshwater tropicals. The genuinely unusual trait, though, is salinity tolerance: wild Molly populations move naturally between fresh, brackish, and full-strength coastal salt water, making them one of the very few common aquarium fish that can be kept across that entire range without special adaptation.

Temperature Range in Detail

Temperature	Effect
Below 22°C	Sluggish, weakened immune response, higher disease risk
24-28°C	Ideal range — active, normal breeding cycle
Above 30°C	Elevated metabolism and oxygen demand; stressful if sustained

Why Mollies Tolerate Salt Water

Molly fish are euryhaline — physiologically equipped to regulate their internal salt balance across a wide range of external salinity, from soft fresh water up to seawater strength. This isn't true of the large majority of aquarium fish, which are strictly freshwater or strictly marine. In practice, most home aquarium Mollies are kept in plain fresh or lightly-salted fresh water — full marine conversion is really only relevant for specific brackish or biotope-style setups, not standard community tanks.

Should You Add Aquarium Salt?

Not required, but common practice: roughly 1 teaspoon of aquarium salt per 10 litres nudges hardness and mineral content toward what Mollies naturally prefer, and they tolerate it without issue. It is not the same as converting the tank to brackish — that's a much larger, deliberate salinity increase for a specific setup, not a general Molly-care step.

Fixing Soft or Acidic Water

If your tap water runs soft and acidic (common in areas with naturally soft water supply), crushed coral or aragonite substrate/media gradually raises both pH and hardness as water flows over it — a simpler, more stable fix than dosing chemical buffers repeatedly. See the main care guide for the full water parameter list alongside tank size and filtration.

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Frequently Asked Questions

What temperature do Molly fish need?

24-28°C (75-82°F). Below that range they become sluggish and more disease-prone; above it, their metabolism and oxygen demand rise faster than most tanks can comfortably supply.

Can Molly fish live in brackish or salt water?

Yes — uniquely among common aquarium fish, Mollies tolerate a wide salinity range, from fully fresh water up to full marine salinity. Wild populations naturally move between fresh, brackish, and coastal salt water.

Do Molly fish need aquarium salt added to freshwater tanks?

Not strictly required, but a small amount (roughly 1 teaspoon per 10 litres) is commonly used to mimic their preferred harder, slightly mineralised water and is well tolerated.

What happens if the water is too soft or acidic for Mollies?

They tolerate it short-term but develop more chronic health issues over time. Buffering with crushed coral or aragonite substrate raises both pH and hardness toward their preferred range.

Do Molly fish need a heater?

Only if your room temperature regularly drops below 24°C. In a consistently warm room, an unheated tank can stay in range without one — check with a thermometer rather than assuming.