During this time goldfish, koi carp etc enter a state of semi hibernation that suppresses their immune system and leaves them vulnerable to bacteria, parasites and illness brought on by stress. Because of their slow metabolism, limit their food to wheat germ and other easy to digest foods.
Ponds will generally have a warm layer of water at the bottom, generated by heat from the underlying ground. At the coldest times when fish become semi torpid, they will sit almost motionless in this slightly warmer layer to help them to get through the worst of the winter. So don’t forget to check the water levels in your pond and top them up as necessary.
If your pond freezes over, resist the urge to hack at the ice, as the shockwave's will concuss the vulnerable fish below. Instead, melt some of the ice with boiling water, and place a floating ball or hoop on the water surface to maintain a vent hole. This vent is necessary to prevent a build up of toxic “marsh gas” caused by decaying organic matter.
Any foliage not cleared in the autumn should be removed now. Around 90% of marginal pond plants can be cut back to water level, and doing so ensures vigorous growth for the coming spring. Any plants that are outgrowing your pond need to be lifted and divided. Any surplus plants left after division can be replanted in another area of your pond, given to friends or relocated to your compost heap.
As oxygenating plants are rampant growers you will probably need to thin these each year. February is a good month to do this before the frogs, newts and toads return to your pond to spawn.
Finally keep an eye out for herons, as the combination of clear water and little plant cover will make your sluggish fish easy pickings. If possible, use anti-heron netting as a last line of defence.
If you haven’t already done so this month is a good time to remove all dead leaves and plant material, and any marginals and Waterlilys that you noticed were spreading too vigorously last year should be divided and replanted.
Frogs, newts and toads will now be retuning to their old spawning grounds (your pond!) so it’s good to get all maintenance work on plants done early so as not to disturb them. Don’t forget that after spawning these little creatures need to be able get out of the water again, so if your pond has steep sides a few boulders acting as stepping-stones will give them a leg up.
The warming water temperatures will begin to rouse your torpid fish, so begin to feed them a little more often. Take the opportunity to check all your pond pumps, filters, pipes, cabling, for frost and any other winter damage.