Topping Up Water Level
Pond liners deteriorate when exposed to the light. When I specified your pond I asked that the liner should be tucked in and up the back of the bricks so it should be possible to keep the pond filled above the liner. The water will seep into the bricks but should not leak through. Water will evaporate and the pond will need topping up. However, do not top up your pond with tap water, as this is dead, inert stuff, with plenty of mineral nutrition for algae but not much else. Therefore, leave it for two to three days for the chlorines to waste away, and a few of the other additives and minerals to drop out. If filling up with tap water quickly is unavoidable then run a hose from a height so it splashes down absorbing oxygen.
Keep the Water Well Oxygenated
There needs to be enough oxygen in the water. Spreading the water out and moving it through the air is important. You need to get as much of the water as possible in contact with the air so it can absorb oxygen. Keep your pump running as much as possible as the aerobic bacteria will die if deprived of oxygen for more than a few hours. For barley straw to work effectively the water also has to be well oxygenated. Barley straw contains a natural algicide and also produces a large number of water fleas that feed directly on the algae. Remove straw at the end of the season.
Ongoing maintenance
As a general rule ponds should be cleaned out as infrequently as possible, because refilling invariably results in green water as the algae proliferates. The process of creating a well-balanced pond then starts all over again. Keep the pond clean by removing excessive dead vegetation and leaves when you see them.
In spring consider replacements of plants in good time. Divide plants that have become too big. Apply barley straw to suppress algae. Service pumps as necessary.
In summer deadhead self seeding plants. Remove algae and dead or dying leaves. Add new plants. Top up water. Clean pumps if necessary.
In autumn cut down and remove excessive growth of plants such as Elodea. Can drain and remove pumps.
In winter take frost precautions. Clear dead foliage.