Water Quality Management in Aquaculture: A Guide

The key water-quality variables in fish farming and how to manage them: dissolved oxygen, temperature, ammonia, nitrite, nitrate, pH, alkalinity, hardness and carbon dioxide.

Overview

Water quality is the set of physical and chemical conditions that determine whether cultured fish stay healthy and grow. In aquaculture a small number of variables account for most production problems: dissolved oxygen, temperature, the nitrogen compounds (ammonia, nitrite, nitrate), pH, alkalinity and hardness, and carbon dioxide. These variables interact, so managing one often affects the others, and routine monitoring is the basis of good management.

Dissolved oxygen and temperature

Dissolved oxygen (DO) is generally the most critical water-quality factor. Texas A&M AgriLife notes that oxygen is naturally low in water, typically under about 10 ppm, that most warm-water fish are stressed below about 3 ppm, and that levels below about 2 ppm can be lethal. Temperature controls fish metabolism and the maximum oxygen the water can hold; warmer water holds less oxygen while raising the fish's demand for it, so high temperature and heavy stocking together increase oxygen risk.

Ammonia

Ammonia is excreted by fish and released by decomposing feed and organic matter. Total ammonia nitrogen exists as un-ionized ammonia (NH3) and ionized ammonium (NH4+). UF/IFAS reports that the un-ionized form is roughly 100 times more toxic than the ionized form, and that its proportion rises as pH and temperature increase. UF/IFAS cites that tissue damage can begin near 0.05 mg/L of un-ionized ammonia and that sensitive fish typically die around 2.0 mg/L.

Nitrite and nitrate

Nitrite is the intermediate product of nitrification, when ammonia-oxidizing bacteria convert ammonia to nitrite and nitrite-oxidizing bacteria convert nitrite to nitrate. UF/IFAS notes that nitrite is toxic to fish and that nitrate is the comparatively non-toxic end product, traditionally considered harmless up to a few hundred mg/L, although newer research suggests effects at high levels. A newly started biofilter takes about 6 to 8 weeks for nitrifying bacteria to establish.

pH, alkalinity and hardness

UF/IFAS states that a pH from about 6.5 to 9.0 is tolerated by most fish, with most natural waters between 6 and 9 due to bicarbonate buffering. Total alkalinity is the water's capacity to neutralize acid and buffer pH swings; SRAC indicates suggested alkalinity of about 50 to 300 mg/L as CaCO3 depending on species and system, and a desirable total hardness of about 50 to 150 mg/L, with greater than 20 mg/L acceptable. Crushed agricultural limestone is added to raise alkalinity where it is low.

Carbon dioxide

Carbon dioxide accumulates from respiration and lowers pH. In ponds, daytime photosynthesis consumes CO2 and raises pH while nighttime respiration releases CO2 and lowers it, producing a daily pH cycle that is buffered by alkalinity. UF/IFAS notes that a pH of 6.0 to 6.5 is unlikely to harm fish unless CO2 exceeds about 100 mg/L. Adequate alkalinity dampens both CO2 buildup and pH swings.

Monitoring and corrective actions

• Monitor dissolved oxygen most often, especially around dawn when pond oxygen is lowest.
• If ammonia or nitrite rises, reduce or stop feeding for one to a few days and exchange water.
• Aerate to raise dissolved oxygen and to strip excess carbon dioxide.
• Lime to raise low alkalinity and stabilize pH.
• Interpret each parameter together with the others, since they interact.