Trouble shooting green water, and why isn't your biological filter working

Biological filters

There are many effective way of cleaning water. What will concern us here will be the most effective way of removing algae, organic matter and the chemical nitrites and ammonia from pond water. We can do this in almost a single process and evolve a state of affairs that is self-sustaining with the minimum of maintenance by what is commonly called a BIOLOGICAL FILTER.

Here is a process in which pond water is pumped through a medium that digests the rubbish in that water. Together with the oxygen that is contained in the water, a colony of bacteria breaks down organic matter (by following the 'Nitrogen cycle') through ammonia, to nitrites, to nitrates that get taken up by the plants. This is what normally happens in the bottom of the pond. Biological filters are like "turbo-version of a pond bottom! And so theoretically with a filter you can keep more fish in clearer healthy water....or can you?

PROS but mostly CONS

The technology of biological pool filters has evolved to satisfy a demand by the pool enthusiast fuelled by his desire to see his fish at all times. The cause of him not seeing his fish is generally single celled plants, collectively known as algae. This is what we imagine the filter is designed to remove. Algae in water are quite natural and harmless thriving in sunlight and lack of competition for the nutrients in the pond water. They are a necessary link in the cycle of life in the pond and provide a vitamin rich soup for the fish. It is ironic that most filters do not remove many of those constituents in the water that are harmful.

Things that can be said against 'Biological pool filters' are:

1. They are expensive to buy - retail - but then you could always make your own.

2. If they are effective, they do in fact cover up what could be inadequate design in the pool specifications.

3. They are merely a stopgap in an overcrowded environment.

4. They cover up mismanagement of the pool environment in respect to the overcrowding and also over feeding.

5. They need to be running all the time - 24 hours a day - and this can create problems if you are going away for a long summer holiday.

6. Some of the 'detritus' that is filtered out could be useful food in the pool particularly for the baby fish and any other beasties of that bottom dwelling world like caddis fly larvae.

7. If you had not planned to have a filter it may be awkward fitting it in. It may look unsightly. Many people have it running into an existing waterfall. But this takes some of the performance out of the pump and does not allow you to maintain the stream, repair it or even just switch the noise of it off.

8. When you resort to a filter, you have become a confirmed fish-keeper. I am not saying there is anything wrong in this, but the water garden as a landscape feature, as another element or dimension in your garden, may be compromised as the passions of the hobby (which can become a way of life) take grip. The welfare of the fish becomes paramount, to the detriment of the rest of the environment - ecologically and aesthetically. Filter systems emerge like power stations in the countryside; alarms and heron tripwires are like pylons carrying their hazardous cables thick and fast.

9. The main instigation for people to purchase a filter is the presence of green water. If the filter is successful at making the water clear, beware of becoming complacent about whether the pond is a healthy environment or not. Therefore....