Itâ€™s mid-summer and, apart from algae growth, your pond will more than likely be looking great. However, it wonâ€™t stay that way without a bit of attention, says Jeremy Gay.
A pond in this beautiful season is what many of us imagine as an idyllic scenario; clear water, luscious pond plants, active fish and lots of insect life. Your pond should be looking its best, the fish their most ravenous — and at last you can get to sit outside and appreciate the fruits of your earlier labours.
As good as your pond gets, however, it may actually be balancing on a knife-edge — and here’s why...
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.
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.
Pond plants are great, but you don’t want too many ‘oxygenating plants’ (pictured above) in the water because of the reversal of gases at night. Aim for surface coverage of about one third and only have more if accompanied by constant water movement or an air pump and air stone.
If you have fish you should have a filter in all but the largest and most lightly stocked ponds. Summertime is when the filter is working hardest, especially if a pressurised filter and UV.
The UV disrupts the green algal cells that cause green water and they get trapped in a fine sponge. In summer the filter is in overtime clearing green water, but that also means more waste being trapped in the media.
Couple that with increased solid waste from increased fish feeding, larger fish as they grow and possible extra fish from spring breeding, and your filter will clog more quickly and need to be cleaned more frequently.
As efficient and compact as they are, I prefer standard gravity type box filters to the pressurised canister types as they clog more slowly — and if they do become clogged, water bypasses easily. Leave a pressurised filter too long and, if it doesn’t come with a bypass as fitted, you lose flow. This means that you also lose oxygenation and filtration to your pond.
I’ve seen many cases of people returning from summer holiday to find their pump and filter had stopped and their fish dead or dying. If you go away a lot and don’t want to maintain your filter on a daily basis in summertime, choose a box filter over a pressurised one.
Warm sunlit water is most prone to algae growth and you will tend to get one of two types of algae, but never together. Green water is exactly how it sounds, with your pond water resembling pea soup. It is actually harmless and usually goes through boom and bust periods as nutrients accumulate, the green water blooming then crashing as the nutrients get used up.
Natural control includes shading the surface of the pond with plants. A combination of aquatic, marginal, deep water and floating plants will not only reduce light streaming through the water but soak up the nutrients the algae needs to bloom.
Another natural method is to add Daphnia, as these minute filter feeders consume green water and clear it. Swan mussels are said to clear it too, though they need perfect water conditions yet a constant food supply to filter out and consume, so good results are not gained by everyone.
Daphnia are a great addition to natural, unfiltered ponds and make for great first foods for fish fry. Add it to green water, then fish fry, and you get a lovely little food chain as the Daphnia eat the algae cells and the fish fry eat the Daphnia. However, a clinical, filtered pond will remove Daphnia quickly, either by filtering it out, having a lack of microscopic food or simply from being over predated by fish.
This is a fairly modern phenomenon, possibly more common now as phosphate levels increase in our mains tapwater. Basically the bane of the pond keeper is to clear the pond of green water with a UV, only to suffer from blanket weed growth in the crystal clear water.
Again it feeds on nutrients, so excess algae growth means there are high levels of either phosphates or nitrates or both, coupled with too much sunlight.
Ultra violet clarifiers (UVCs) are not effective on blanket weed as it grows on the sides of the pond instead of being waterborne and passes through the filter and UVC.
Algae also produce oxygen and consume CO2, yet do the opposite at night. Too much algae is like having too many plants and they may strip oxygen levels. So, apart from looking unsightly, algae can also cause breathing problems for your fish.
Furthermore, oxygenate heavily when using algae-killing liquids and powders as oxygen may be stripped from the water. Test the water, as if significant algae dies off it can foul the water. Previously, as unsightly as it was, it may have actually been benefiting the reduction of nutrients, not adding to it.
I’m not talking protein skimming here, as this is more the domain of the top-end Koi ponds. This is surface skimming; a simple practice that can aid gaseous exchange, lighten mechanical and biological load and generally make your pond look nicer.
Surface skimmers can be adapted filter inlets which draw water across the pond, scaling down to simple skimmer nets used regularly to remove floating debris by hand.
The surface of a pond can build up with all sorts of rubbish in summertime: dead insects, pollen, foaming from use of algae treatments and proteins and oils from heavy feeding. Floating duckweed (pictured above) is a particular pain. It is a tiny green plant that plagues some ponds, suffocating everything beneath it and becoming a real risk to oxygen levels. It’s a good nutrient filter but that is about it, and too much looks unsightly.
Install a surface skimmer and all these problems go away, quickly and effectively. A clean surface free of debris goes back to that all-important element— oxygen — so by skimming the surface, keeping the water moving and keeping an eye on planting and algae you help to prevent a heatwave wipe-out.
If your fish look to be suffering, the last thing to do is add food. This will add to the biological demands of the entire pond and cause the oxygen-consuming bacteria to take more of it. Fish will not eat if too hot, with uneaten food causing pollution — or they will eat, excreting more ammonia from their gills as they demand more oxygen. In short, in a heatwave, don’t feed.
Try to shade the pond, keeping it cool long term. Floating ice or a massive water change aren’t brilliant ideas, as a sudden change to cooler water will often bring on whitespot through stress.
If yet to dig your pond, site it away from full sunlight as this will cause a rise in temperature and nuisance algae. Dig it as deep as possible, as a deeper pond will stay cooler in summer and warmer in winter, yet will fluctuate much less.
If you think of the shape of your pond in profile, try not to create a shallow, satellite dish as, although it will have a high surface area, this will also mean temperature fluctuations and lots of algae growth.
This item was first published in the August 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.