PHIL CALVERT: How to keep your garden pond clean

Nowadays, however, he can hear me coming down the stairs and as I enter the room he is ready and waiting for me and then trots over to the patio door to be let out into the garden. Strangely, however, this is never for the purpose of relieving himself. He rarely drinks anything and despite water always being available in a bowl on the kitchen floor, he never bothers to have a drink.

Consequently, he pops out onto the patio area, stretches himself in a slow but purposeful manner and then, more often than not, just sits there as if taking in the air of the new day. If he takes a drink at all, it is now. It is a close choice between the green water of a plant saucer, or the delights of the water in our garden pond

I can imagine a human being very ill if he or she drank from similar sources, but he seems to come to no harm. I suppose he is just being a dog, and as a species dogs are not renowned for their social skills, etiquette or air of refinement. After refreshing himself in this disgusting water, he usually sits down again for a thorough scratch before returning to his bed for another hour before we leave for work.

As the pond seems to be his only source of drinking water, I have been increasingly conscious of the deteriorating state of the water over the past week. I gave the pond itself a little bit of a post-winter tidy earlier in the spring but recently, we have seen an algal bloom developing in the water. It is a common enough problem during May. As temperatures rise, clear water starts to fog-up with tiny single-cell algae which have the potential to turn the pond an unpleasant pea-soup green if left unchecked.

Apart from the unpleasant, neglected-looking appearance, there can be a negative impact on oxygen levels in the water, which can put fish stocks under stress. Before you reach the pea-soup stage, it is good practice to take remedial action.

I like to carry out a partial water change which not only gets rid of a little of the green water algae, it also removes pollutants from the water caused by the accumulation of fish waste and decaying plant and leaf material. It also gives the pond a look of having been freshened up.

Much more important is to use something to clear the rest of the algae. Barley straw is great for helping to prevent green algae blooms, but I have had only limited success. The most obvious answer is to use a chemical algicide, perhaps the most popular of which, are those that act as a flocculant and clump the microscopic algae into larger particles which can be screened out by the pond filter or which will sink to the bottom to be consumed by beneficial bacteria or removed with a pond vac.

The latter is my preferred form of attack as by getting the pond vac to clear up sunken algae, you inevitably bring out silt, dead leaves and other potential water pollutants. It is a strangely satisfying chore.

Obviously, however, the effect is only temporary, and no treatments provide a permanent solution, for that you will need a filter. A basic pond filter performs two functions. It picks up various bits of debris sent to it by the pump, and, secondly, it performs a biological function. Fish waste, like any animal waste, is high in ammonia and if left unprocessed will build up in the pond. A healthy filter is packed full of beneficial bacteria, which make the various filter media their home. They colonise the filter, breed and multiply, and thankfully, convert the fish waste from toxic ammonia to less toxic nitrite and eventually to the much more benign nitrate, which just happens to be plant food.

It is, therefore, our microscopic bacterial allies which are the key to keeping a pond healthy and in balance. Thoroughly cleaning your filter with tapwater, which is loaded with chlorine, is one of the worst things you can do as whole colonies of these good guys are wiped out. If there is a build-up, remove the sponge or some of the media and rinse in a bucket of pond water and then put them back still a little dirty to continue their excellent work. Not cleaning the media at all, however, is not a long-term solution, so a gentle rinse to keep things sweet every couple of months is best.

Most filters include an ultraviolet clarifier which is irrelevant to the good bacteria but which massacres the green water algae. These are not essential if your pond is a correctly balanced environment of water, bacteria, plants and not overstocked with fish. They are essential with heavier fish stocks but the biological action is the key.

So I recently cleaned out my pond to give it a new lease of life and topped it up with fresh water (treated with a dechlorinator to protect my bacteria) and watched with interest. The water was clearer and the fish moved about contentedly. I had placed some new oxygenating plants in the bottom to help keep things sweet. The new marginal plants around the edges improved things visually and in the centre a small raft of floating plants provided shade in anticipation of the expected hot summer sun!

Monty greeted me and I let him outside. He stopped in his tracks. Things were not right. He sniffed here. He sniffed there. He looked with contempt at the now clean pond as if some vandal had trashed the place. Clearly the pond water was now no longer fit to drink in his view. No scratching or stretching today as there were more important issues on the agenda. Defying months of regular habit he drank nothing but methodically annointed every bit of the garden. Running on empty and with his work complete he trotted inside to his cushion for another hour in bed, apparently content that order had been restored. I dread to think what would happen if we changed his cushion!