Using rainwater in an aquarium

That rainwater is soft is the easy part, and every city water guide on this site already says so. What those guides do not cover is the part that actually goes wrong: what dissolves off a roof on the way to the tank, and what to put back into the water once you have it. This guide is about those two problems and nothing else.

Published 1 August 2026 · Last updated 1 August 2026 · 9 min read

Why raw rainwater is not aquarium water

Rainwater arrives with essentially no dissolved minerals. General hardness and carbonate hardness both read at or near zero, which means there is nothing holding the pH steady. In a tank, the acids produced by normal biological filtration then push the pH down with nothing to resist them, and the result is a slow drift punctuated by sudden crashes after water changes.

The second consequence is biological. Shrimp and snails build their shells and exoskeletons from calcium and carbonate taken from the water. In near-zero GH they cannot, which shows up as failed moults in shrimp and pitted, eroded shells in snails. Fish tolerate soft water better than invertebrates do, but soft-water tolerance is not the same as zero-mineral tolerance.

The city water guides for Adelaide, Perth and Brisbane each cover how rainwater interacts with the local mains supply, and the water parameters guide explains GH and KH properly. This guide assumes you have read one of those and moves on to the parts they do not cover.

What comes off the roof

Rain is clean when it lands. It stops being clean the moment it touches a roof, and a rainwater tank is really a sample of everything on the catchment surface, dissolved and concentrated. Most of it is harmless to a household drinking supply after treatment and still matters a great deal to an aquarium, because fish live in the water rather than drinking a glass of it.

Metals

This is the serious category. Zinc leaches from galvanised iron roofing and older galvanised guttering, and is toxic to fish and invertebrates at low concentrations. Lead comes from lead flashing around chimneys and vents and from lead-headed roofing nails, both still present on many older Australian houses. Copper comes from copper guttering, downpipes and any copper plumbing in the collection path, and is the one to fear most: copper is lethal to shrimp and snails at concentrations far below what harms fish, which is why every shrimp guide warns against copper-based medications.

Coatings and timber

Bitumen and tar-based waterproofing membranes on older roofs shed hydrocarbons. Copper-chrome-arsenate treated timber in a pergola or verandah draining into the same gutter contributes copper, chromium and arsenic. Neither belongs upstream of an aquarium supply.

Organic load

Bird and possum droppings, leaf litter and accumulated dust all wash into the tank and carry an ammonia and bacterial load with them. This is the least dangerous category and the easiest to manage, since it is concentrated in the first runoff of a rain event and is exactly what a first-flush diverter is designed to remove.

By roof material, modern coated steel such as Colorbond and glazed tile are the safest catchments. Unglazed concrete tile is acceptable and leaches a small amount of lime, which in this context is a mild benefit rather than a problem. Bare galvanised iron, any surface with lead flashing, bitumen membrane and copper are the ones to rule out.

First-flush diverters

A first-flush diverter is a simple device fitted to a downpipe that captures and discards the first portion of runoff from each rain event before the remainder enters the tank. It exists because contamination is not spread evenly through a storm: the initial flow carries the great majority of the accumulated dust, droppings, leaf debris and dissolved metals, and the water arriving ten minutes later is dramatically cleaner.

The usual design is a vertical chamber that fills with the first runoff and seals itself with a floating ball once full, diverting subsequent flow into the tank. A small drip outlet empties the chamber slowly between events so it is ready for the next one.

Sizing depends on the connected roof area, and manufacturers publish tables for their own products. Fit the diverter according to that guidance rather than a rule of thumb, and if you are choosing between two sizes, take the larger one. Over-diverting costs you a small amount of water; under-diverting defeats the purpose of fitting it at all.

Two habits matter as much as the hardware. Keep gutters clear, since a gutter full of decomposing leaf litter contaminates every rain event regardless of what is fitted downstream. And discard the first rain after a long dry spell entirely, because months of accumulated deposition comes off in one go and no diverter is sized for it.

Testing rainwater before you trust it

Test a sample from the tank itself, not from a bucket left out in the rain, since the stored water is what you will actually use. Expect GH and KH at or near zero and a pH reading that is low and unstable, and treat any pH result on unbuffered water as approximate: with no carbonate present, the reading moves with handling.

The test that matters most is one an API Freshwater Master Test Kit does not include. If there is any copper anywhere in your catchment or plumbing, buy a copper test kit and use it. It is the only way to detect the contaminant most likely to kill an invertebrate tank.

There is also a practical biological check that costs nothing. Fill a jar with your remineralised rainwater, add two or three ramshorn or bladder snails from an established tank, and leave it a week. Snails are more sensitive to dissolved metals than fish are and cheaper to risk. If they are alive, active and showing no shell pitting after a week, the water is very likely fine. If they are lethargic or dead, something is in there that a hardness test will never show you.

Remineralising: the recipe

The goal is to put back the general hardness and carbonate hardness the rain does not carry. For a general tropical community tank, aim for roughly 4 to 6 dGH and 3 to 4 dKH. That is soft water by the standards of most fish, buffered enough to be stable, and comfortable for the large majority of community species.

Carbonate hardness is straightforward and cheap. Plain sodium bicarbonate, sold as bicarb soda, raises KH with no complications:

General hardness wants both calcium and magnesium rather than calcium alone, in roughly a three to one ratio. A worked example for 100 litres of rainwater, targeting about 4 dGH and 3 dKH:

• 9 g bicarb soda for 3 dKH
• 9 g gypsum for 3 dGH of calcium hardness
• 4.4 g Epsom salts for 1 dGH of magnesium hardness

The simpler route is a commercial remineraliser. Seachem Equilibrium provides balanced GH without adding KH, which suits this job well since the two are dosed separately here anyway. Follow the label rate for your target hardness rather than converting from the figures above, because product formulations differ.

The working routine is: fill the drum, add the salts, circulate with a powerhead or air stone for an hour until fully dissolved, test GH, KH and pH, adjust if needed, then use. Once you have found the doses that hit your targets with your water, it becomes a repeatable measurement rather than a calculation.

Blending, the easier alternative

Remineralising from zero gives complete control, and most people do not need it. If you have mains water available, blending is simpler, cheaper and harder to get wrong: the tap water supplies the minerals and the rainwater dilutes them to the level you want.

Blending genuinely helps in the harder supplies. On a Brisbane or Adelaide connection, or in the limestone corridor north of Perth, cutting mains water with rainwater brings hardness down to suit soft-water species without touching a single chemical. In Melbourne or Sydney, where the mains water is already very soft, blending achieves little and remineralising the mains supply is the more useful move.

Treat the blend as you would treat tap water: dechlorinate the full volume, since the mains portion still carries chlorine or chloramine. Seachem Prime handles both, which matters in the chloraminated supplies where standing the water does not help.

When rainwater is worth the trouble

Rainwater earns its keep in three situations. If you are on a hard supply and want to keep soft-water species, it is the cheapest dilution available. If you are on a bore, it is usually the only practical way to bring hardness and nitrate down, which the bore water guide covers in more detail. And if you are breeding species that require genuinely soft water, controlled remineralisation from a zero baseline gives precision that no tap supply can match.

It is not worth the trouble if you are in Melbourne or Sydney with a standard community tank. That water is already soft, already suitable for most community fish, and adding a collection, testing and remineralising routine to it buys nothing. The guide to the best fish for Australian tap water is the better starting point if your goal is simply to stock what your existing supply already suits.

Related: aquarium water parameters explained · using bore water in an aquarium · keeping freshwater shrimp · hardness converter · the species directory.