WATER CHEMISTRY

WATER CHEMISTRY

Tropical freshwater aquarium fish originate from a diverse range of aquatic habitats, including fast-flowing mountain streams, expansive lakes, and slow-moving rainforest rivers. Each of these environments has unique water properties influenced by surrounding geological and biological factors. Because fish are adapted to specific natural water conditions, their captive environment should closely mimic these parameters. To successfully replicate natural habitats, aquarists should have a basic understanding of water chemistry—specifically, pH, water hardness, and the nitrogen cycle.

pH
The pH scale measures the acidity or alkalinity of a substance, ranging from 0 to 14. A pH of 7.0 is neutral, meaning it has an equal concentration of hydrogen ions (H⁺) and hydroxide ions (OH^-). Water with a pH below 7.0 is considered acidic (higher H⁺ concentration), while water with a pH above 7.0 is alkaline or basic (higher OH^- concentration). Most freshwater fish thrive in a pH range of 5.0 to 9.0, with many species preferring slightly acidic to neutral conditions (6.0–7.5).

In aquariums, tap water pH can be adjusted using phosphate, bicarbonate, and other buffering agents. However, pH may still fluctuate due to biological activity. A decline in pH is often caused by the accumulation of organic matter, such as decomposing plant material and fish waste. If the pH drops below 5.5, the beneficial bacteria responsible for breaking down ammonium (Nitrosomonas) become less effective, leading to an increase in ammonium levels.

Lowering the pH: pH can be lowered using commercial pH-reducing chemicals available at pet stores. However, many of these products contain phosphate, which promotes algae growth. Another natural way to lower pH is through respiration—fish and plants release carbon dioxide (CO₂), which forms carbonic acid and lowers pH. In hard water, lowering pH can be challenging. First, soften the water through peat filtration or reverse osmosis, then attempt pH adjustments.

Raising the pH: pH can be increased by adding baking soda (sodium bicarbonate) or by performing water changes to remove accumulated organic acids.

Water Hardness

Water hardness measures the concentration of dissolved minerals, primarily calcium (Ca) and magnesium (Mg). Hardness is expressed in terms of calcium carbonate (CaCO₃) and measured in parts per million (ppm), carbonate hardness (kH), and general hardness (gH or dH). Water is classified as soft or hard based on the following scale:

For most aquarists, water hardness is not a major concern. However, extremely soft water has poor buffering capacity due to low CaCO₃ content. This can lead to pH crashes, where the pH rapidly declines, harming aquarium inhabitants.

Carbonate Hardness (kH): Carbonate hardness refers to the concentration of carbonate (CO₃) and bicarbonate (HCO₃) ions in water. While kH is often associated with overall hardness, it specifically impacts pH stability by buffering against sudden fluctuations.

Adjusting Water Hardness: - To soften water: Use peat filtration, reverse osmosis, or ion exchange resins. Boiling water for an extended period can also reduce hardness. - To harden water: Filter water through dolomite or crushed coral to increase mineral content.

Electrical Conductivity: The electrical conductivity of water is determined by the presence of dissolved ions. Harder water has a higher conductivity due to increased mineral content. Conductivity tests measure the total concentration of ions but do not specify their composition (e.g., Mg, Ca, Fe).

Oxygen
Aquatic plants, animals, and bacteria rely on dissolved oxygen for respiration. Oxygen can be introduced into an aquarium through air pumps with air stones, filtration systems that agitate the water surface, or wet-dry filter setups. During the day, plants contribute oxygen via photosynthesis, but at night they consume oxygen through respiration. Warmer water holds less dissolved oxygen, so additional aeration may be necessary at higher temperatures.

Carbon Dioxide (CO₂)

Carbon dioxide is a natural byproduct of respiration in fish, plants, and bacteria. During photosynthesis, plants utilize CO₂. Insufficient CO₂ can cause plant leaves to yellow and slow their growth. Some dissolved CO₂ forms carbonic acid, lowering pH levels. However, excessive CO₂ can suffocate fish.

Adding CO₂: Carbon dioxide can be supplemented by softening the water (to free CO₂ from calcium bicarbonate) or through specialized CO₂ injection systems.

The Nitrogen Cycle

When fish excrete waste, plants decay, and uneaten food decomposes, nitrogenous compounds accumulate in the water. These compounds do not disappear but are broken down by bacteria into progressively less toxic forms through a process called nitrification. This cycle is especially noticeable in newly established tanks or when a tank is out of balance.

In a new tank, nitrifying bacteria are initially absent unless gravel or filters from an established tank are introduced. Hardy fish are added to produce waste, which generates ammonia (NH₃) or ammonium (NH₄). If the pH is above 7.0, toxic ammonia predominates; if the pH is below 7.0, less toxic ammonium forms. Within a few days, ammonia levels rise to harmful concentrations.

As the cycle progresses, Nitrosomonas bacteria develop and begin converting ammonia into nitrite (NO₂), which is still toxic but less so than ammonia. Subsequently, Nitrobacter bacteria convert nitrite into nitrate (NO₃), which is the least toxic nitrogenous compound. Nitrate is absorbed by plants and algae or removed through water changes.

The Nitrogen Cycle Process:
Nitrosomonas Nitrobacter
NH₃ (Ammonia) → NO₂ (Nitrite) → NO₃ (Nitrate)
Excess food & fish waste → Broken down by bacteria → Absorbed by plants or removed via water changes

If waste accumulates due to overfeeding, overstocking, or infrequent water changes, ammonia levels can rise again, causing stress or death in fish. To restore balance, perform water changes and monitor parameters.

This cycle can also be disrupted if the beneficial bacteria are killed, such as when a tank or filter is washed with soap or disinfectants. In such cases, the cycle must restart. Water with a pH below 5.5 or certain medications can also reduce bacterial populations, necessitating careful water management.

Maintaining a well-balanced nitrogen cycle is essential for the long-term health of an aquarium, ensuring that waste is efficiently processed and removed.