Common Garden Pool Problems

Pests of water plants

Probably the most troublesome pest of aquatic plants is the waterlily aphis, for not only does it attack nymphaeas, but all manner of succulent marginals as well. Looking very much like the notorious black bean aphis, this little pest breeds at a prodigious rate, smothering leaves, stems and flowers and causing widespread disfigurement.

The breeding habits and complicated life cycle of this pest are worth studying. However, it is sufficient to say here that reproduction takes place continuously on the host plants during the summer months, followed by mass migration to nearby plum and cherry trees during early autumn. Here eggs are deposited on the boughs where they over-winter, the emerging generation next spring returning to the water plants.

Obviously control of any insect infestation in a pool which is stocked with fish is likely to be difficult. All that can be recommended is washing the pests into the pool with a strong jet of clear water and hope that the fish will clear them up. Some will doubtless climb back onto the foliage, but if the process is repeated regularly they will be kept under control.

Spraying neighbouring plum and cherry trees with a winter tar oil wash will considerably reduce the over-wintering population. It also follows that when planning a pool, consideration should be given to the placing of trees of this family within the surrounding garden. Never, for instance, use one of the weeping flowering cherries at the poolside. Although it may well look delightful, it will be impossible to keep aphis-free.

Waterlily beetles can also be troublesome, but are fortunately only of local occurrence. The small dark brown beetles and shiny black larvae will be found on both foliage and flowers of waterlilies where the latter strips them of epidermal tissue. This results in infection and subsequent decay.

Again, forcibly spraying with clear water will dislodge the pests, but it is equally important to remove the dead remains of poolside plants during the autumn as it is here that waterlily beetles take refuge during the winter.

Both the beautiful china mark moth and brown china mark moth can cause considerable damage to aquatic plants. The little brownish and orange moths look inoffensive enough, which indeed they are. It is their caterpillars that cause all the trouble. These cut and shred the foliage, those of the beautiful china mark moth even having the audacity to burrow into the stems as well. Not content with cutting up foliage, they then proceed to stick pieces together and form a shelter in which they can live in comparative safety.

Even if it were possible to spray with insecticide they would be fairly secure. The only method of control that is effective is handpicking. This is a long and tedious business, especially if the infestation is severe. Under such circumstances it is better to remove the foliage from all deep water aquatics and net off all pieces of floating plant debris. Hopefully most of the larvae will be removed and the plants can make fresh healthy growth from the base.

Caddis flies have larvae which live in a similar manner, constructing shelters from all sorts of pond debris and in this relative safety feeding on all kinds of aquatic plants. There is very little chance of effecting a control, either manual or chemical, for apart from being sturdily built their shelters are well camouflaged. Fortunately, fish consider such larvae a delicacy and providing the pool is adequately stocked they should cause very little trouble.

False leaf-mining midge sometimes makes an appearance in the pool and when it does the effect can be devastating. The tiny larvae attack the floating foliage of deep water aquatics, eating a narrow tracery of lines all over the surface. These usually remain unnoticed until the damaged parts become infected and the entire leaf rots and collapses. This can be particularly serious for less vigorous waterlilies. Forcible spraying of the foliage immediately damage is noticed will control the pest, but by this time the foliage is unlikely to recover.

Aquatic plant diseases

Fortunately, few diseases affect water plants, but those that do are particularly destructive and difficult to control.

Leaf spots are the most frequent assailants with two distinct species attacking the foliage of waterlilies, but causing similar damage. This appears in the form of dark patches on or around the edges of the leaves, which eventually rot through and lead to the collapse of the foliage. Both are very infectious and spread through a pond like wildfire, so immediately trouble is spotted the infected plants should be removed and destroyed. Spraying with a weak solution of Bordeaux mixture in the absence of fish will contain the disease, but unless a rare or special variety is infected, treatment is not worthwhile.

With waterlily root rot the same applies. In fact, nothing can be done to arrest this disease once it takes a hold, but the waterlilies that remain in the pool can be protected by impregnating the water with copper sulphate. A small quantity of crystals being tied up in a muslin bag and dragged through the water until dissolved. All fish must be removed before doing this as copper sulphate is toxic. Personally I would never attempt such treatment in an ordinary garden pool. It is far better to remove and destroy the waterlilies and do without them for a season rather than risk the lives of the fish.

Waterlily root rot is not difficult to diagnose, for affected plants have leaf and flower stems which become soft and blackened, and evil-smelling roots which turn gelatinous. An altogether unsavoury disease which is usually introduced with freshly purchased plants. So never plant a waterlily that has a soft or blackened area on its rootstock. Even if it turns out not to be root rot it is better to be safe than sorry.

When the pool turns cloudy I am not going to propose any magical cure-all for a dirty or green pond, rather am I going to suggest the reasons for it and leave the pool owner to take what action he sees fit. To put not too fine a point on it, except in its initial stages of establishment, the degree of uncleanliness of a pool reflects to a considerable extent the degree of mismanagement by its owner. I do not mean to be harsh, but having dealt with this particular problem from pool owners over a number of years, I find that the greatest obstacle to putting matters right is the questioner’s belief that he cannot possibly have done anything wrong, and that there is a special chemical that can be added to the water which will give instant clarity. If the pool owner is prepared to humble himself and read on, then he is well on the way to correcting the trouble.

There are three main kinds of cloudy water. Green water, which is caused by suspended algae; muddy water, which is brought about by the distribution of soil particles; and evil-smelling milky water which is a sure sign that something is decaying on the pool floor.

Let us look at the last condition first. This is directly attributable to decaying organic matter within the pool and is extremely serious, causing rapid oxygen depletion and loss of fish. Only by emptying the pool and removing the offending pollutant can this condition be cured. In a small pool the occasional loss of a sizable fish and its subsequent decomposition may cause the trouble, but more often than not it is due to the accumulation of dead leaves from surrounding trees. Netting the pool at leaf fall will prevent the trouble recurring.

Dirty or muddy water might not be so easy to cure. Especially if the pool is natural or the plants have been planted directly on to the marginal shelves or the pool floor. Fish stirring up mud in their search for insect larvae are the usual culprits, tench and carp being particularly troublesome.

The only way to cure such a problem is to ensure that all exposed areas of soil are covered with a generous layer of pea shingle. This prevents the fish from nosing about and stirring up the soil, yet allows them to withdraw delicacies such as gnat larvae. When the plants are grown in baskets this is easily done, but if they have been planted directly into soil the entire pool must be emptied before the gravel can be distributed.

If we now turn to green water we find the commonest and most frustrating problem the pool owner encounters. As suggested earlier, green water is brought about basically by bad management. Temporary opaqueness in a pool where the plants have not become established is quite normal and eventually corrects itself. There may also be a green algal bloom in the water during the first warm days of spring owing to the aquatic plants not having surged into growth. But permanently green water needs more plants.

I am not going to rewrite the theory of natural balance here, but it is essential to have sufficient submerged oxygenating plants to compete with the algae for mineral salts. About a third of the surface area must also be covered by floating foliage in order to reduce the amount of direct sunlight falling beneath the water and thus make life intolerable for slimes and algae. This is the only way to permanent clarity.

Algaecides are useful in achieving a balance, but are not a substitute for it. I am generally against the widespread use of chemicals in horticulture, but there is a case for using them in a newly established pool to kill off rapidly growing green algae. The algae reduce the intensity of sunlight beneath the water and, therefore, the speed of growth of the all important submerged plants. The same applies to a mature green pond where additional submerged plants are being introduced. Plunging containers full of oxygenating plants into murky green water will eventually effect a cure, but by destroying the algae chemically at the outset the plants will get a much better start and soon take control of the situation.

Common fish disorders

The pests and diseases that affect decorative fish are legion. Indeed, if we look at the diversity of ills that can befall a fish it is surprising that any have survived. Put into perspective though, they are dangers with which we should concern ourselves, but not serious threats if a good standard of pool hygiene is maintained and all new additions to the pool are carefully vetted.

Describing all the ailments that might befall fish in a garden pool would not only be tedious, but extremely depressing. So what I will do now is mention those that are likely to be encountered, together with early warning signs that something might be wrong. Correct treatment at an early stage will save fish from all diseases, except fish tuberculosis, and can help considerably when they have been attacked by insect predators.

Undoubtedly the most common problem the pool owner will have to contend with is fungus in all its various forms. This manifests itself as a white cotton wool-like growth which is usually a secondary infection on open wounds, although fish run down after the winter may develop it in patches on seemingly healthy parts of their bodies.

If treated as soon as noticed fungus is not a serious malady. But if allowed to continue unchecked, is a slow killer which will gradually spread through a pond. Dipping affected fish regularly in a sea salt solution over a period of time usually brings about a cure, but modern fungus cures based on methylene blue and malachite green used in the same way result in a much speedier recovery.

Malachite green can also be used to cure that other unpleasant fish ailment, fin rot. Its popular name describes precisely what this bacteria does. Destroying fins and tails, and if not treated promptly, eating into the body of the fish as well and causing an extremely unpleasant death.

White spot disease is caused by a group of single celled creatures called protozoa which for part of their life cycle become embedded in the flesh of a fish. On their departure they leave deep pock marks which are open to infection by fungal diseases. Not only that, but the cycle is recurring and if left unchecked the parasites will re-infest the fish in ever increasing numbers until it eventually becomes weakened and dies.

At the stage in which the parasite is embedded in the fish it is fairly safe. However, once it bores its way through the skin and becomes free-swimming it is extremely vulnerable and can be easily killed by methylene blue or else quinine salts such as quinine hydrochloride and quinine sulphate. Such treatment can only be provided for fish in small volumes of water and is both impractical and undesirable to attempt in the open pool. The best way of ridding a pool of the parasites is to leave it devoid of fish for six weeks. During that period of time all free-swimming parasites have to find a host or perish.

Several pests attack fish, some of which are hideous in their cruelty. The anchor worm, for instance, not strictly speaking a worm, but a brutal crustacean. This has a slender tube-like body less than 1cm long with a horrible barbed head which it imbeds in the flesh of its host. During its murderous feast lesions and unsightly tumours appear which eventually become infected with fungus, leading to the eventual death of the fish. Affected fish must be captured and held in a net while the anchor worm is dabbed with a solution of potassium permanganate or domestic paraffin. This kills the creature, which can then be withdrawn with tweezers and the open wound treated with a mild household disinfectant.

Fish lice perform in a similar manner, clinging to the body of the fish, especially in and around the gills. They are strange creatures with almost transparent flattened bodies and gruesome feelers with which they attach themselves to their host. Paraffin dabbed on their bodies dislodges them, but it is important to dip the fish in a malachite green solution before returning them to the pond as some of the fleshy tissue will have doubtless been damaged.

Leeches sometimes attack fish, although the majority of species prey upon snails. Dragonfly larvae can also cause trouble, as well as great diving beetles and water scorpions. None of these can be regarded as a serious threat though, for they will only attack an occasional fish and do not warrant any control measures. Beetles, leeches and scorpions are all part of pond life and we should accept them as such. A lost fish now and then is a small price to pay for the added interest these creatures can give a water garden. And after all it is just Nature’s way of ensuring survival and maintaining a balance between the various life forms.

Before leaving the problems that might afflict fish, it is perhaps appropriate for me to mention the most humane method of killing an ailing fish. As will have been appreciated, most disorders can be treated with a reasonable amount of success if spotted early enough. However, some cases will have gone too far before they are discovered and on certain occasions the treatment may appear to cause extreme discomfort with only a slight chance of recovery. In these cases, and when a fish is carrying a highly contagious disease, it is best destroyed.

The fish to be disposed of should be taken in a dry cloth and then dashed smartly against a hard surface such as a concrete path. Death will be instantaneous.

Coping with the heron

Apart from all the problems that may occur within the pool, there are those that may quite unexpectedly come from outside. Of these the heron is likely to be the most troublesome.


It is well known that herons wade into the water and stand waiting for their unsuspecting prey. So small mesh netting can be spread out across the pool to stop their antics. Unfortunately, this not only looks unsightly, but causes all kinds of problems with the plants growing through it and eventually becomes an awful tangle.

The best deterrent I have found is a row of short pieces of cane, no more than 10cm high, placed at regular intervals around the perimeter of the pool. Attached to these is a strong black thread or fishing line. When the heron advances towards the pool his legs come in contact with the thread and he will go no further. After two or three sorties he will almost invariably skulk away.

04. August 2011 by Dave Pinkney
Categories: Water Features | Tags: , , | Comments Off on Common Garden Pool Problems


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