At the beginning of the calendar year the water garden shows little sign of life or colour, except for the bright green surface foliage of Callitriche uerna grown in shallow water. This is when we shall be glad of the winter-flowering heathers planted on the adjoining rockery and borders. Fish lie low and take no food. They need far less oxygen, too, in cold weather and they do not suffer through the inactivity of oxygenating plants. They do not suffer from cold either – directly, that is – but they can suffer fatally if the cold is severe enough to freeze the surface over. There is always some decaying vegetation in a pool; the by-products of decay include suchgases as methane and hydrogen sulphide. The quantities may be quite small, and normally they escape without doing harm, but if a layer of ice prevents this they can build up to a level sufficient to poison the fish. This might take a week but could happen more rapidly if the pool contains a lot of decaying and plant remains. It is essential, therefore, to keep a hole open in the ice at all times.
It is useless simply to break the ice. Heavy blows on thick ice produce concussive shock waves in the water. And, of course, the hole will quickly freeze over again. The same applies to a hole made by pouring hot water into a can standing on the ice. Another possibility is to make a hole by the hot-water-in-the-can method, and then syphon out enough water to leave a 2- or 3-in. Gap between ice and water surface. In theory the ice produces aeffect that prevents a fresh layer forming on the water surface. In practice I find that the ice usually collapses and the surface is soon frozen over again.
The simplest, and by far the most reliable, way of keeping that vital hole open – and keeping it open night and day, whether you’re at the office or in bed -is to use a pool heater. This, in effect, is a small immersion heater with a float. It uses little more current than an electric light bulb and it is on the job all the time; as long as it is switched on the pool cannot freeze over completely. In large pools several may be desirable. Let me emphasise, however, that the purpose is not to heat up all the pool water to a nice cosy temperature for the fish, but simply to keep a small area free of ice, and in most cases one heater is adequate to do this. In any pool equipped with a pump to operate a fountain or waterfall “(which won’t be needed in the winter anyway) it is a very simple matter to disconnect the pump and connect up a pool heater in its place.
The other harmful effect of ice is the damage it can do to concrete pools, particularly if they have vertical sides. The pressure of expanding ice can split the sides of a concrete pool as easily as it bursts water pipes. This damage can be avoided by floating in the water such compressible objects as old tennis balls (if you have a quantity – one is no use), beach balls, logs or planks (nothing treated with creosote or chemicals, of course) or polystyrene packing boxes ballasted so that they don’t just ride on top as the ice forms. As these expendable items yield to the pressure of ice expansion they relieve the strain on the pool walls.