As good as your pond gets, it may actually be balancing on a knife-edge — and here’s why. In the summer when the water heats up, oxygen levels drop. As a result pond fish struggle to breath and can suffocate.

Oxygen
Oxygen, or the lack of it, is the biggest risk to your fish in summer. On the plus side you have aquatic plants, pond pumps and maybe an air pump. Among the negatives are the reduced ability of warm water to hold oxygen, increasing demands of your fish, and the fact that aquatic plants actually demand oxygen at night instead of producing it.

The signs
How do you know when your pond is short of oxygen? You may not know until too late when you look one morning after a hot, sticky night and some of your fish have gone belly up. If you find that fish have died overnight for no apparent reason, it is more than likely through lack of oxygen. What’s more, it will often just be the larger fish that go as they demand more of it. Another giveaway is when you just lose Golden Orfe, as they are the most oxygen demanding of the pond fish commonly available.

How to fix it
Aeration is the key to increasing oxygen levels and driving off harmful CO2. Pond water can be aerated by moving it with a pump — a fountain attachment or waterfall are best, though beware that turning these off at night often causes problems. Leave these fountains or waterfalls running 24/7 in hot weather. An air pump designed just for pond use is ideal. The reason why many people turn off fountains and waterfalls is not so much to save energy as to reduce noise at night. If so, consider a pond air pump and one or more large air stones as they will efficiently aerate at a noise level that won’t be audible. Aeration from air stones can also help with pond filtration. Filter bacteria are aerobic and the more oxygen you provide them with the more efficiently they do their job. Drop an air stone into a pond box filter and it may be able to cope with more waste.