Water quality judgment techniques and methods
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"Fish cannot live without water, and fish farming depends on the quality of water. These common sayings highlight how crucial water quality is to successful aquaculture. To develop a productive fishery, it's essential to have clean, nutrient-rich water that supports fish growth and reproduction. The levels of organic and inorganic substances, the presence of natural food sources like plankton, and various physical and chemical properties of the water all play a vital role in determining fish health and productivity.
Water, much like land, can be 'fat' or 'thin.' The term 'fat water' refers to the richness of nutrients, particularly the abundance of plankton, which serves as a primary food source for fish. Since plankton has color, its concentration directly affects how much sunlight penetrates the water. In practice, water fatness is often judged by its clarity—measured by transparency.
The method for measuring transparency involves using a Secchi disk. When no disk is available, you can estimate it by holding your hand palm-up at arm’s length, slowly submerging it into the water until it disappears. The depth at which this happens gives an idea of the water’s clarity. Ideal transparency for fish ponds typically ranges from 20 to 40 cm, with 25–30 cm being optimal.
Water color is another key indicator of its quality. It reflects the presence of dissolved substances, suspended particles, plankton, and even the sky and bottom. Based on color, water can be categorized into several types:
- **Thin water** appears light green or blue-green, with high transparency (60–70 cm) and few plankton, but more filamentous algae.
- **Bad water** has many phytoplankton species, but they may form a slimy or fibrous layer that’s hard for fish to digest.
- **Cloudy dark gray or blue-green water** indicates high turbidity, often due to excessive cyanobacteria, which are not easily digested.
- **Green water** is thick, with low transparency and a yellow-green film, often leading to water aging if not managed.
- **Red water** features clusters of red zooplankton, signaling poor water quality that needs immediate adjustment.
- **Brown or oil-green water** is generally favorable, containing diatoms and other easy-to-digest plankton.
- **Red-brown or brown water** contains harmful toxins and should be avoided.
- **Industrial pollution water** shows unusual colors and contains heavy metals, making it unsuitable for fish farming.
The concept of “fat, live, tender, and cool†water represents ideal conditions for aquaculture:
- **Fat**: High plankton content, around 30 cm transparency.
- **Live**: Water changes color and transparency regularly, showing active plankton.
- **Tender**: Fresh and easy to digest, avoiding overgrowth of difficult-to-digest species.
- **Cool**: Clear, with high oxygen levels and minimal surface film.
To maintain these conditions, regular feeding and fertilization, along with water exchange, are necessary. Aquaculturists also use the "four looks" method to assess water quality: observe color, check for color changes, examine wind films, and look for algal blooms. Each of these indicators helps determine whether the water is healthy and suitable for fish farming."