Local water

Using bore water in an aquarium

Every city water profile on this site ends with the same caveat: none of it applies if you are on a bore. Groundwater is not a variation on mains water, it is a different substance, and the one thing that can be said about it generally is that it varies enormously between properties. It is also the only water in Australia that is reliably, genuinely hard, which makes it an asset for the right stock and a serious problem for everything else.

Published 1 August 2026 · Last updated 1 August 2026 · 10 min read

Test before anything else

Mains water comes with a published profile because a utility tests it continuously and reports the results. Bore water comes with nothing. Two properties on the same road can draw from different depths into different aquifers and produce water that shares no meaningful characteristic. This is not a case where a guide can tell you your numbers.

Before a drop goes into a tank, test the raw water for general hardness, carbonate hardness, pH, ammonia and nitrate. An API Freshwater Master Test Kit covers pH, ammonia, nitrite and nitrate, and hardness kits are sold separately. Test a sample that has been drawn and left to stand for 24 hours as well as a fresh one, because bore water changes markedly on contact with air and the standing sample is closer to what your fish will actually experience.

If the property has a laboratory analysis from when the bore was drilled or last certified, that is worth digging out. It will usually report iron, manganese, sulfate, chloride and total dissolved solids, which are all things aquarium test kits do not measure and which matter here.

Hardness and pH

Groundwater picks up minerals from the rock it sits in, and in most of Australia that means calcium and magnesium carbonates. The result is water that is hard on the aquarium scale rather than merely mineral-rich, frequently above 300 ppm GH (about 17 dGH) with carbonate hardness to match and a pH between 7.5 and 8.5.

This is worth stating plainly because it is unusual. No Australian capital has genuinely hard mains water: Brisbane averages around 121 ppm, Adelaide 108 ppm, and Melbourne and Sydney are far softer again. Bore water is the exception, and the only supply in the country that suits rift lake cichlids with no adjustment at all. The water parameters guide covers the hardness scale in full, and the hardness converter moves between ppm and dGH at the standard 1 dGH = 17.86 ppm.

High carbonate hardness has a practical upside that soft-water keepers envy: the pH is held very firmly. You will not get the pH crashes that catch out keepers on Melbourne or Sydney water. The trade-off is that you cannot easily lower the pH either. Peat, driftwood, botanicals and pH-down products all fail against a strong carbonate buffer, and fighting it is a losing game. Stock to the water instead.

Iron

Dissolved iron is the classic bore water signature. In the ground it is held in a soluble form and the water looks clear at the tap; on contact with air it oxidises and drops out as the orange-brown precipitate that stains fixtures, decor and filter media. Keepers usually notice it as rust-coloured deposits on sponge filters within a week.

Iron at the levels typical of Australian bores is not directly toxic to fish. The problems are cosmetic and mechanical: staining, clogged filter media, and a persistent tint in the water column. It also fertilises algae, so an iron-rich supply combined with strong lighting produces algae faster than the same tank on mains water.

The fix is to let it happen before the water reaches the tank. Aerate the raw water vigorously in a holding drum for 24 hours, which oxidises the iron, then let it settle and draw the clear water off the top, leaving the sediment behind. Running it through a filter sock or fine mechanical media at that point removes what remains. This costs nothing but time and a spare drum.

Hydrogen sulfide

The rotten egg smell that some bores produce is hydrogen sulfide, formed by bacteria in oxygen-poor groundwater. It is genuinely toxic to fish, and unlike iron you can detect it without a test kit.

The good news is that it is volatile and comes out of solution readily. The same aeration step that handles iron removes hydrogen sulfide, usually within a few hours. If the smell has gone after a day of vigorous aeration, the sulfide has gone with it. If it has not, the bore has a persistent bacterial issue that is worth addressing for the household supply and not just the aquarium.

Nitrate, the one that actually kills fish

Iron stains and sulfide smells, so both get attention. Nitrate does neither, and it is the reason to take testing seriously.

Groundwater under cropped or grazed land accumulates nitrate from fertiliser and animal waste, and agricultural bores can deliver water at 50 ppm nitrate or higher before it has been anywhere near a fish. That matters because the entire purpose of a water change is to dilute accumulated nitrate. If your source water already carries more nitrate than your tank does, water changes make the problem worse rather than better, and a keeper doing everything correctly watches their nitrate climb with no explanation.

There is no practical way to strip nitrate from bore water at household scale. The realistic options are dilution with a low-nitrate source, or heavier planting so the tank consumes nitrate biologically. Fast-growing stem plants and floating plants do consume meaningful quantities, though not enough to rescue a supply reading 60 ppm.

Salinity and total dissolved solids

In parts of Western Australia, South Australia and inland New South Wales, bore water carries a high dissolved salt load. Total dissolved solids above 1,000 ppm are common in some districts and well above anything a freshwater aquarium should see.

Moderately elevated TDS is tolerable for hard-water species and livebearers, which handle it better than most. Genuinely saline bore water is not usable for a freshwater tank at any dilution that leaves it worth the effort, and if that is your supply the honest answer is to use rainwater as the primary source and treat the bore as the thing you blend in for minerals rather than the other way around.

A treatment sequence that works

Putting the pieces together, this is the routine that handles most Australian bore supplies:

Draw and aerate. Fill a holding drum and run an air stone in it for 24 hours. This oxidises iron, drives off hydrogen sulfide, and equalises temperature.
Settle and decant. Turn the air off, leave it an hour, and draw the clear water from the top. The iron precipitate stays on the bottom.
Test the treated water, not the raw water, for GH, KH, pH and nitrate. These are the numbers your fish will actually meet.
Blend if the numbers demand it. Rainwater is the obvious diluent in most of rural Australia and brings both nitrate and hardness down at once. See the rainwater guide for the contamination risks worth knowing before you use roof water.
Match temperature and add nothing else. Bore water needs no dechlorinator, because there is no chlorine or chloramine in it.

That last point cuts both ways. No chlorine means no chemical treatment step, but it also means the water has no residual disinfection and can carry live bacteria from the aquifer. The aeration and holding step handles this in practice, which is another reason not to run bore water straight from the outlet into a tank.

Fish that suit bore water

Hard, alkaline, strongly buffered water is not a compromise, it is the natural condition of some of the most popular fish in the hobby. Working with it produces better results than any amount of chemical correction.

Rift lake cichlids are the obvious fit and the genuine opportunity, since keepers everywhere else in Australia have to build this water artificially. The electric yellow cichlid and demasoni cichlid both want exactly what a limestone bore delivers. Livebearers are the other natural group: guppies, platies, mollies and swordtails all do better in hard, alkaline water than in the soft water most Australian cities supply. Nerite snails and other shelled invertebrates also benefit, since they need carbonate to build shell and suffer visible erosion in soft water.

What to avoid is equally clear. Soft-water specialists will survive in hard water but never colour up or breed: cardinal tetras, chili rasboras, German blue rams and most wild-type Apistogramma belong in this group. Anything sold as a blackwater species is a poor match for a bore.

For a fuller treatment of matching stock to Australian water in general, the guide to the best freshwater fish for Australian tap water works through the same logic across every supply type.