41 · Filter Media & UV Clarifiers
The box you buy matters less than what is inside it. Here is how to choose media and when UV actually helps.
Mechanical vs biological
Mechanical filtration removes solids (foam, brushes, sieves). Biological filtration houses the bacteria that convert ammonia and nitrite. You need both; skimping on mechanical loading just clogs the bio-stage.
Bio-media comparison
| Media | Surface area | Notes |
|---|---|---|
| Lava rock | Good | Cheap, dense, can trap sludge |
| Bioballs / Kaldnes (K1) | Very high | Moving-bed; needs flow to tumble |
| Sintered glass / ceramic | Highest | Premium price, excellent colonization |
| Matting / fleece | Moderate | Best as mechanical pre-filter |
UV clarifiers — the honest view
UV kills suspended algae cells as water passes the lamp, clearing green water. It does not remove the nutrients that caused the bloom, and it can also knock back beneficial plankton. Use UV as a targeted fix for green-water outbreaks, not as a substitute for plants and balance.
Pressure vs gravity filters
Pressure filters (with built-in UV) sit buried and push water uphill — convenient but harder to clean. Gravity filters sit above the pond, self-emptying, and are easier to maintain at scale.
| Media | Surface area | Cleaning ease | Cost |
|---|---|---|---|
| Japanese matting | High | Easy | Medium |
| K1 Moving Bed | Very high | Self-cleaning (in moving bed) | High |
| Bio-balls | Medium | Easy | Low |
| Volcanic rock | High | Difficult (clogs) | Low |
| Ceramic rings | Very high | Moderate | Medium |
- Clean the quartz sleeve every 3 months with a vinegar solution.
- Replace the UV bulb every 8–12 months (even if it still glows, germicidal output drops).
- Check for cracks in the quartz sleeve — water ingress will destroy the ballast.
- Size the UVC for your pond volume: 4W per 1,000 litres (green water), 2W per 1,000 litres (maintenance).