Nitrates are the natural by-product of the breakdown of ammonia and nitrites in a pond environment. While a small amount of these substances is safe, if they remain unchecked they can become very harmful to your fish.
High nitrate levels can cause various problems for your fish including brown-blood disease, a potentially fatal disease that prevents red blood cells from accessing vital oxygen.
Having lots of pond plants will go a long way to keep your nitrate levels in check, as well increasing aeration and doing partial water changes.
Envii Nitrate Klear is an ideal solution to reducing nitrate levels. It uses bacterial strains which target the nitrates and nitrites and work to remove them. It works as low as 8°C which means you can treat for most of the year.