Rainwater and Australian native fish

Australian freshwater fish and rainwater

If you ask Google the question is rainwater good for fish the broad answer is NO

Rainwater is not good for fish, particularly Australian native fish, and in many cases, it is dangerous.

It is low in nutrients, lacks minerals necessary for fish to survive and grow.

It is soft, slightly acidic, and lacks essential minerals GH/KH

It could have pollutants from the roof as it ran off.

Fish require nutrient-rich, stable water, not rainwater, which is better for plants.

It is unstable because the pH is not fixed.

Key considerations for rainwater used for fish
Low nutrient content

Unlike fish tank waste which provides nitrogen (ammonia/nitrates), rainwater lacks the nutrient density necessary for fish health.
Mineral deficiency

Rainwater lacks calcium and magnesium (low GH/KH), which are essential for fish health and pH stabilization.
Contamination risks

Rainwater collected from roofs can contain debris, oils, or contaminants.
Safety requirements

If used, rainwater often needs to be conditioned, filtered, and remineralized.

Fish cannot survive in pure water.

Not that rainwater is "pure" but it is pretty close.

It is almost distilled water - demineralised.

The main issue is the pH of rainwaterand the almost complete lack of necessary minerals and trace elements absolutely necessary for the survival of freshwater fish.

Rainwater is acidic, typically ranging from a pH of 5.0 to 5.6.

This acidity is naturaly caused by atmospheric carbon dioxide being dissolved in the raindrop as it falls through the atmosphere.

Not to be confused with acid rainwhich has a pH of 4.2 to 4.4.

The lack of minerals makes it impossible for fish to osmoregulate.

Fish will die from their own body chemicals leaching out into the water.

Osmoregulation in fresh water fish is how they keep a balance of salts in their bodies.

Just like humans, salt is essential for survival.

In pure water lacking in salt or any minerals, salt will move from one to the other, from the fish to the water, to try to keep a balance.
This is called osmoregulation.

That's why freshwater fish can't live in salt water.

They can't keep the balance in such a high salt concentration.

In rainwater its the same problem but in reverse - no salt at all.

It's not all about salt.

Calcium and magnesium, which make up water hardness, are essential in the biological processes of aquatic animals, for example, bone and scale formation in fish. Dolomite is one easy way to fix this.

Dolomite is a common calcium-magnesium carbonate mineral that should be easy to buy.

It is similar to limestone but contains more magnesium.

It is used in agriculture and even in the construction industry.

The critical component of total hardness is the calcium concentration, or calcium hardness.

See this webpage

And this webpage for the professional (Members area access only.)

More about pH here

And this page about conductivity

pH and conductivity is a relatively reliable way to quickly measure the mineral content of water, as well as actually measuring the water hardness.

I use the instruments below.

pH electronic tester that also measures conductivity