A warning that waste is being produced faster than the biological filter can process it.
POND WATER QUALITY
The water can look clear and still be unsafe.
Regular testing tells you what fish behaviour and water clarity cannot. Keep a written record so changes are visible before they become emergencies.
The biological cycle in plain English
Fish waste and foodproduce
Ammoniaprocessed by bacteria
Nitriteprocessed by bacteria
Nitrateused by plants and diluted
Ammonia and nitrite are toxic. A new or damaged filter may not yet contain enough bacteria to process them. Overstocking, heavy feeding, loss of circulation, medication, chlorine exposure or cleaning the filter too aggressively can all overwhelm the system.
What to test
Can rise after ammonia as the bacterial population develops. It interferes with oxygen transport in fish.
Usually less immediately toxic but useful for tracking long-term nutrient accumulation.
pH stability matters. Carbonate hardness helps resist sudden pH changes; interpret results together.
New pond syndrome
OATA warns that stocking too quickly can create unhealthy ammonia and nitrite because there are not enough nitrifying bacteria. Add fish gradually, keep the filter running, test the water and respond to the measured cause rather than pouring in several treatments at once.
If fish are gasping or distressed
Increase aeration, check pump and filter circulation, test immediately and seek experienced aquatic or veterinary advice. Do not assume that green water is the only problem.
Adding or changing water
Tap water disinfectants can harm fish and filter bacteria. Use a conditioner that treats the disinfectants used by your water supplier and dose for the new tap water being added. Simply standing water may remove free chlorine but does not reliably remove chloramine.
Cleaning the filter without restarting it
Clean mechanical sections often enough to maintain flow. Treat biological media gently, use pond water for rinsing where the instructions permit, and never clean every mature surface to laboratory brightness. Keep media wet and oxygenated during maintenance and restart circulation promptly.
Green water, blanket weed and UV
Green water consists of suspended algae; blanket weed grows in strands or mats. A correctly sized UV clarifier can help with suspended green water when flow and lamp performance are correct, but it does not remove the underlying nutrient load. Remove trapped waste, control feeding, improve plant balance and verify that the UV lamp is the correct genuine type and within its service life.