Why blooms matter
A healthy algal bloom is the pond's primary DO producer, shade layer, and secondary food source. A crashing bloom is the pond's fastest killer — dead algae consume oxygen and release ammonia in the same 24-hour window.
Secchi depth: reading the bloom
Lower a Secchi disc until you lose sight of it; measure the depth. Log daily at mid-morning. Target 30–45 cm for shrimp, 25–40 cm for tilapia. Trends matter more than snapshots — a drop from 40 cm to 20 cm in three days means the bloom is thickening fast and a crash is likely.
Good bloom vs bad bloom
Green-brown blooms dominated by diatoms and green algae are desirable. Bright green blue-green (cyanobacteria) blooms are trouble — toxin-producing, prone to sudden crashes, and often paired with off-flavour in harvested product.
Preventing bloom crashes
- Maintain alkalinity above 80 mg/L CaCO₃ to buffer pH swings that trigger crashes.
- Add carbon (molasses, rice bran) to feed heterotrophic bacteria and control cyanobacteria.
- Partial water exchange (10–20%) once Secchi drops below 25 cm.
- Avoid algaecides — controlled crashes are always safer than uncontrolled ones, and safer still is not dosing.
Turbidity from clay and silt
Not all cloudy water is algae. Clay turbidity blocks light without producing oxygen, starving the pond of both photosynthesis and visibility. Diagnose by leaving a sample in a clear jar — clay settles; algae stays suspended.
Response protocols
- Secchi < 20 cm → likely bloom crash risk → aerate, exchange 10%, halt feed 24 h.
- Secchi > 60 cm → bloom too thin → add fertiliser (urea + DAP 3:1 at 5 kg/ha).
- Clay turbidity persistent → gypsum 200–400 kg/ha + grass dike stabilisation.