Many older koi ponds share the same story: the water stays green and smells, and a few days after a water change it is back. The problem is usually not the water but the filtration.
Green water is algae
Sunlight plus excess nutrients in the water (fish waste, uneaten food) lets algae multiply and turn the water green. With strong Malaysian sun, ponds with little shade and weak filtration are especially prone to it.
Good filtration does three jobs
- Mechanical filtration: traps fish waste, leaves and leftover food so they can be removed regularly.
- Biological filtration: beneficial bacteria living on the filter media break down ammonia from fish waste into less harmful compounds. This is the key to stable water and needs enough media and time to establish.
- Algae control: for example a UV clarifier, together with some shade and aquatic plants.
Common problems in older ponds
- The filter is too small for the pond size and number of fish.
- No bottom drain, so waste settles and rots on the floor.
- The pump is undersized or water flow leaves dead zones that never reach the filter.
- The fish have grown bigger and more numerous, but the filtration was never upgraded.
Do not change too much water at once
Replacing most of the water at once disturbs the established bacteria, and sudden changes in water quality stress the fish. Small, regular water changes with good filtration work better.
When should you get it checked?
If the water turns green again soon after a change, smells, your fish often gasp at the surface, or the pond is losing water, have the filter, pump and pond structure checked. Often the pond does not need rebuilding; upgrading filtration and drainage makes a clear difference.
We build koi ponds and water walls, and upgrade filtration and fix leaks in older ponds. Book a site visit and we will take a look.
2026-10-08