Above-ground fish pond

It is the detail that determines whether maintenance actually gets done. A partial water change is the most frequent task in a fish pond, and when it involves a bucket or a siphon, it tends to get put off. Here, you open a tap, let out the volume you want, and top the level back up. The water you remove is not wasted either, rich in nitrates, it makes an excellent watering for a border or a vegetable patch in open ground. Refill slowly and with water at a similar temperature, since it is the thermal shock from refilling, far more than the water change itself, that puts stress on your fish. A renewal of around one tenth of the volume per week suits most setups, and can be adjusted from there based on what a nitrate test tells you.

In a system where fish, bacteria and plants share the same water, this tank is the starting point of the circuit. It is from here that the pump draws water towards the growing beds, and it is here that the water returns, stripped of its nitrates. Before adding a single fish, you must allow the bacteria that convert ammonia nitrogen into nitrites and then into nitrates to become established, this constitutes the nitrogen cycle and takes several weeks, during which the pump and aerator run in an unpopulated system. You should introduce your first fish only once both ammonium and nitrite have fallen back to zero while nitrate is rising, and not a day sooner, however impatient you may feel. A rushed start results in losses, and it is by far the most common mistake made with a first installation.

The round shape is not merely a by-product of the construction method. Water circulates rather than stagnating in corners, and sediment gathers in the centre rather than spreading everywhere, making cleaning far simpler than in a rectangular tank. Direct the water return along the wall rather than towards the centre, and this circular flow maintains itself without any additional pump output. Take advantage of each partial water change to run a soft sponge along the wall at the waterline, where deposits build up first and are most visible. This way you will keep the walls clean without ever needing to empty the tank completely.

A collapsible tank can be dismantled, and that is its other advantage over a dug pond. If you are not overwintering fish, drain it in autumn, let the liner dry out completely before folding it, and store it away from frost and rodents, which are quite happy to gnaw through folded PVC in a garden shed. If fish are to spend the winter in it, the situation changes, since an above-ground volume cools far more quickly than a sunken pond and can freeze to a considerable depth. In that case, plan for peripheral insulation and keep an aeration point free of ice, or transfer your fish for the cold season. Many users run this tank seasonally only, from April to October, which simplifies everything and considerably extends the life of the liner.