In a typical pond you could not have too much aeration, in certain circumstances you can get what's called 'super saturation' which can be very dangerous to fish. Super saturation occurs when air is added to water under pressure, in a pond system this could happen if you had a pin hole in a pipe leading to a pump, as the water is drawn through the pipe air is pulled in through the hole, however this is quite rare.
More likely in a pond you can waste oxygen by pumping too much in via an air pump, oxygen is very important in a pond, particularly if it is heavily stocked for fish and in particular koi. In a traditional garden pond you may have a waterfall and some oxygenating plants, as these may add enough oxygen into the water. However koi are quite large fish, some as long as a metre and would quickly eat any plants in the pond, also the large filtration units used on koi ponds require large amounts of oxygen in order to work, so you will quite often have to add additional aeration to both the pond and filtration system, remember the warmer the water the less oxygen will be available, ponds with excess algae growth can also suffer from lack of oxygen as the algae uses it up.
However this is where many people start to add too much air, it will not do any harm but it can be unsightly seeing a Jacuzzi type water swirl, and most of the air evaporates back into the atmosphere rather than dissolving in the pond water.
Tips for Adding Aeration to Your Pond Filter
Pond filters use large amounts of oxygen in order for the filtration process to work, remember a pond filter is a 'living microclimate' with bacteria converting harmful pollutants in the pond water into harmless nitrate which evaporates, in most pond filters the easiest way of adding extra air is by using a suitable air pump with some air stones, place the air stones at the bottom of the filter media, as a rough guide use one air stone for every square foot, for large filters you may use larger air disks or air rings, not only will the air bubbles help feed the bacteria, but it will also stop solid waste settling, helping to keep your filters cleaner, so a few extra bubbles here will not do any harm.
Tips for Adding Aeration to Your Koi Pond
This is where I often see excess use of aeration, if you place a large 20cm air disk for example in you pond with a 40 litre air pump attached, you are probably straining the pump which will shorten its life, and at least 50% of the air you are putting in the pond will come straight back out at the surface of the pond.
All air disks and stones have a limit to how much oxygen can be pumped through, when you pump too much air into them you will generally find that the bubbles are larger, the finer the bubbles the more oxygen will be dissolved into the pond water, as an example if you used a 20 litre air pump on the 20cm air disk, the result would be less disturbance pond surface and more oxygen dissolved into the pond water. For a pond of around 3000 gallons a 20 litre air pump and suitable air disk or if preferred around 6 standard air stones would provide more than adequate aeration into your pond.
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