Well Water for Aquariums: Gases, Iron, Hardness and Testing

Groundwater from private wells varies by site and season. How dissolved CO2, H2S and nitrogen, iron, manganese, hardness and farm nitrate affect fish, and how to test and degas it.

Why well water varies

Well water is groundwater that has spent time in contact with soil and rock, so its chemistry reflects local geology rather than a treatment plant. The Southern Regional Aquaculture Center (SRAC) notes that groundwater is the primary water source for aquaculture in its region, and that its alkalinity and hardness are set directly by the bedrock and soils it has passed through. Because the water is underground and isolated from the atmosphere, it is often low in oxygen and can carry dissolved gases. SRAC also stresses that groundwater parameters fluctuate over time, so a single test is not enough.

A private well is not regulated in the way a public supply is. The US Environmental Protection Agency (EPA) places responsibility for testing on the well owner and recommends testing at least once a year for total coliform bacteria, nitrates, total dissolved solids and pH, with extra tests when local groundwater problems are known, when the well is repaired, or when the water changes in taste, colour or smell.

Dissolved gases: CO2, H2S and nitrogen

• Carbon dioxide: CO2 from plant respiration and oxidation of organic carbon can accumulate in groundwater. High CO2 lowers the blood pH of aquatic animals and reduces the oxygen their blood can carry, whatever the oxygen level of the water. As CO2 rises, pH falls; as it is removed, pH rises. SRAC gives an ideal concentration below 15 mg/L.
• Hydrogen sulfide: H2S forms when bacteria use sulfate under very low oxygen, and decaying organic matter near the source raises it. SRAC states that any detectable amount can harm or kill many aquatic animals and that the target is zero. It has a rotten-egg smell; University of Georgia Extension puts the odour threshold at about 0.5 ppm and notes that it corrodes iron, steel, copper and brass.
• Nitrogen gas: well water drawn from depth is under higher pressure, which keeps gases in solution. When pressure drops at the surface, nitrogen comes out of solution and can form bubbles inside fish if they are exposed too early, causing gas bubble disease. SRAC recommends keeping total gas pressure below 110 %.
• Oxygen: groundwater often has little dissolved oxygen; if CO2 or H2S is detectable, oxygen is probably low, but it can be low even when they are not. SRAC gives a preferred range of 4–8 mg/L (80–100 % saturation).

Iron and manganese

Iron and manganese minerals dissolve readily in the low-oxygen conditions of groundwater; University of Georgia Extension describes this as a natural property of the aquifer, not contamination. Dissolved (ferrous) iron is colourless, but once the water is pumped up and meets oxygen it oxidises to ferric iron and settles as reddish-brown sediment, while manganese forms dark specks. The EPA secondary standards are 0.30 mg/L for iron and 0.05 mg/L for manganese, and iron bacteria that form slimy growths in plumbing can appear at concentrations as low as 0.01 ppm. For aquaculture, SRAC regards iron below 0.1 mg/L as unlikely to be a problem; higher levels increase maintenance, can damage equipment and may worsen existing health problems in the animals. Where iron is high, SRAC advises removing it before degassing and biofiltration.

Hardness, alkalinity and pH

Hardness is the concentration of divalent ions, mainly calcium and magnesium. The US Geological Survey (USGS) classifies water as soft at 0–60 mg/L as CaCO3, moderately hard at 61–120, hard at 121–180 and very hard above 180, and reports that hard water is widespread where aquifers are rich in carbonate rock. Alkalinity, mostly carbonate and bicarbonate, is the water's capacity to absorb acid before pH drops. SRAC gives general aquaculture ranges of 75–200 mg/L CaCO3 for alkalinity and pH 6–9 for most aquatic animals, while noting that many species, especially when breeding, need softer or more acidic water. Because CO2 depresses pH, a sample measured straight from the tap can read lower than the same water after aeration.

Nitrate and other contaminants

The USGS identifies fertiliser and manure on farmland, sewage effluent and runoff as the main sources of nitrate in groundwater, and notes that contamination is more of a problem in shallow aquifers. The EPA lists nitrate and nitrite from fertilisers, septic systems and animal waste among the typical well contaminants, together with microorganisms, heavy metals such as copper, lead, arsenic and cadmium, organic chemicals including pesticides and solvents, radionuclides and fluoride. SRAC adds that groundwater can already contain ammonia, that low background ammonia causes chronic problems in flow-through systems, and that copper and zinc are toxic to aquatic organisms. High salinity or chloride in a freshwater well can indicate seawater intrusion or industrial and agricultural waste.

Testing and preparing well water

1. Obtain a laboratory analysis through a certified lab or the local health department, covering at least the parameters the EPA recommends plus iron, manganese, hardness, alkalinity and copper.
2. Repeat tests over different seasons, because groundwater chemistry changes over time.
3. Hold the water in a separate tank and aerate it vigorously; splashing and air contact release H2S and CO2 and add oxygen. Aquaculture facilities use packed-column degassers, in which water trickles over media against a rising air current.
4. Let any oxidised iron or manganese settle or filter it out, then re-test pH, hardness and ammonia on the aerated water.
5. Adjust hardness and alkalinity only after degassing: calcium or magnesium raise hardness, bicarbonate raises alkalinity, and blending with softened or reverse-osmosis water lowers both (SRAC).