How to Reduce Nitrate Levels in a Garden Koi Pond
QUESTION ... We re-vamped our pond last year. It is 650 gallons, with a waterfall and a fountain, and the water is turned over through a filter and a UV unit. I did a nitrate test for three days running, which was at a dangerously high level. I did two partial water changes (using Freshstart), but it was still high. On the fourth test it was down a little but still unacceptable. If I put plants in the pond the Koi eat them. All I have in there now is a large arum lily which covers nearly half the surface. The fish are often under the lily, can you advise how we can get the nitrate level down?
Mrs W Witchett, Essex.
ANSWER ... It would be interesting to know how many fish you have in your pool and how big they are and how much you feed them. These are all the crucial factors in the level of pollutants in your pool. The fact that there are a high level of nitrates and hopefully no nitrites or ammonia, shows that the biological filter is working breaking down organic compounds and ammonia to these nitrates, which would normally be taken up by plants and used for growth.
Because there are no plants in your pool I expect algae has taken advantage of the nitrates instead, making the water quite green. Funnily enough, the fish will be quite enjoying this vitamen rich soup. In fact the fish can tolerate quite high levels of nitrates, up to 500mg/litre, which is pretty rich!
In order to bring nitrate levels down you need either you need a much more sophisticated filter that can employ anaerobic bacteria that will break up the nitrates into the constituent parts of nitrogen and oxygen (can be difficult - not my department) or employ nature like I do. Try planting up the stream with something like Water Cress or Fools Cress. Bed small plants into corners in a bed of peagravel. Perhaps you could even set up another filter system - a vegetable filter system like your biological filter with the lid off and the filter medium planted up with the likes of the Common Reed (Phragmites australis). These plants are the best for the job.
Also try Viresco