Recently we have had hot and dry weather - the odd torrential downpour has helped the level of water in the pond as it was getting precariously low. Topping up the pond in the height of summer is always a 'should I, shouldn't I' question.
The wildlife in the pond will adapt to the changing level of water and if they want to some creatures will leave. Young will bury themselves deep into the pond and hide. Top up with rain butt water if you can but to avoid losses tap water is better than none at all. The volume of tap water you will be topping up with should be small in proportion to the total volume of water in the pond so done little and often should not alter the water quality too radically at any time.
The plants on marginal shelves need to be able to reach water with their roots or they will die. So in order to maintain the life of the plants and keep the cover around the emergence zone of the pond you may need to top up the pond water to reach the shelf.
Most shallow marginal shelf plants (shelf 0) will live with their roots in water even if their crowns are exposed in the summer months. They will need more protection from water over their crowns in the winter but I am sure the rainfall will sort that out by then.
More problems are found in fish ponds as fish are dependent on enough water to swim in and for their oxygen levels to be maintained so if you have fish that are gasping at the surface you need to add an aerating appliance that will pump oxygen into the water and the water level must be maintained.