Why Pond Depth Matters
Depth of Pond for Fish
The depth of your koi pond will matter in cold weather. If it’s really shallow and cold, the pond is going to freeze in cold climates. Deeper water is going to be less likely to freeze, so you if you are in a cold area of the country, you should consider making you pond deep. Also, koi, when fully grown and depending on which type, grow to about 2 feet in length. Total number of koi is going to determine how much total inches of fish in a koi pond. If you are planting 5 koi you could have a total of 120 inches of fish, so you’re going to need 1200 gallons of water. Depth is going to be part of that calculation.
To calculate the gallons of water in the pond:
Multiply average length x average width x average depth x 7.48
Thus, 10 long x 6 wide x 3 average depth x 7.48 = 1346.4 gallons of water.
You could put about five koi in this size of pond.
Ponds freeze from the top and solid ice can get fairly thick, perhaps six or more inches. Koi will spend winters towards the pond’s bottom, and deeper ponds can help their health. Also, changes in temperature are stressful, so a deeper pond is advised. Water depth will also protect koi from birds, and where the climate is hot in summer, koi might even get sunburned from the hot sunlight. Again, deeper ponds are better overall.
For non-“backyard ponds,” that is, ponds that stock large game fish, you ought to have some deeper spots than is needed for a koi pond. If you have bass, your pond ought to have a minimum of 6’ of depth somewhere in the pond. If possible, try to have areas of 8-12 feet deep. It is best that lots of oxygen spreads around in both winter and summer months.
For catfish, ideally the pond slopes from about 2 feet to 6 feet at the sides. Catfish will like it there.
As you can see, best pond depth is going to vary according to what the pond is being used for.
Why You Need Aeration
Regardless of the above needs, aeration is needed in all ponds. Aeration occurs naturally, but most ponds do need extra help to fight stratification (layering). Pond aeration is needed for health of your fish, who need enough oxygen to thrive. Also, even if you do not have fish, the health of the pond can suffer from lack of aeration, causing the rotten egg smell, algae, filthy water, and excessive algae growth. Aeration keeps the water from stratifying, reducing muck at bottom of pond, pond scum at the top, and reduces invasive plants. For koi ponds with fish, you need to recirculate your water once an hour. For large game fish ponds, you will also need to aerate, though on a larger scale.
Pond Depth and Aeration
The most common type of aeration for large ponds is surface aeration. Surface aeration is also the pretty kind, that is, it is the use of pond fountains. Typically, the pump is attached to a float so that the nozzle rises and lowers with the water level. The pump agitates the water below, decreasing stratification. In addition, water is forced into the air so that the pond’s water is always being recirculated.
There are two common kinds of fountains used for surface aeration. Also, you can choose pure surface aeration with no display whatsoever.
These provide the versatile patterns and typically (not always) include multiple nozzles to change displays when you wish. However, the nozzles can be intricate with many holes, and pump impellers are used. Fewer gallons per minutes of water pushed into the air is the result when compared to aerating fountains, which use a propeller instead of an impeller and only display the classic V-pattern. The volume of water going into the air is much higher. In contrast, surface aerators are only that, aerators. A propeller is used without a nozzle of any kind. You have seen these kinds of aerators at sewage ponds, as they are the most efficient surface aerators, but they provide only a bubbling display.
For very deep ponds (over 7' or 8’), bottom aeration is recommended. Typically, an air compressor is located at the shore and a hose carries the air to a diffuser unit at the bottom of the pond. It is very similar to the air bubbles rising from the bottom of an aquarium.
Again, how deep should your pond be? It depends on what you are trying to accomplish.