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Since I apparently thought last night would lead us directly into Wednesday morning (and even argued with James about it), I find myself in the odd position of being two days early with my blog posting. I can almost guarantee you that it will never happen again. What to do on a gray Tuesday when I have already run on about loving another person?

Well, I thought I would share a little love of the issue of koi and goldfish ponds. Granted, your results will not be the same as my own…

Almost a year ago, my sister took me to a Chinese buffet where we fed the koi. These koi were the most beautiful fish I had ever seen: butterfly koi, I was later informed. After several months of waiting for perfect conditions, we had a pond, a lovely fantail goldfish and a lacy black/white koi. After a couple of weeks, we determined that the pond could sustain itself, with no aeration. After all, the number of plants in the water were sufficient to oxygenate the water. Right?

Well: yes and no.

We have several varieties of water plants: some of which are highly aggressive. Yes, plants do provide oxygen for your fish pond in the daylight hours. However, I was not aware of the photosynthetic reversal that occurs at night. In the dark, the plants take oxygen from the water!

We have had a hot July, folks. Despite keeping the pond shaded and our best efforts (we thought) to oxygenate the pond through plant life–we lost our beguiling koi. Its name was Saxifrage. Considering these fish can live for a couple dozen years, I was quite guilt-stricken. After some quick research, I came across these words of wisdom by Chris Neaves:

Oxygen levels can fluctuate dramatically during the day and night as well as during periods of high and low temperature. There can be significant oxygen variations in ponds with poor circulation or in ponds which do not move the water away from the bottom of the pond i.e. the point furthermost from the atmosphere. There is less oxygen in pond water at higher altitudes – about 18 to 20% less than at the coast. There is continual competition in the pond for the limited amount of oxygen available at any given time. The fish, the plants, the micro organisms all need oxygen rich water – all the time. Algae and submerged plants have a dramatic influence on oxygen levels in a pond during a 24 hour day / night cycle. The photosynthesis process during sunlight may rocket oxygen levels to saturation point and beyond. However, a dramatic plunge in oxygen with the reversal of the photosynthesis process at night can spell disaster, even to the point of fish suffocating in ponds at dawn.

It has been found that if the oxygen levels are 25% below optimum levels first thing in the morning, growth rates are reduced. The turn-over rate will have a direct bearing on oxygen levels as will the stocking densities of fish. The faster the turnover rate the more water will come into contact with the atmosphere and the more gaseous exchange will take place. The more fish in the pond, the less oxygen in the water, as they are breathing all the time.

Higher temperatures in summer mean there is less oxygen that can be dissolved into the water. higher temperatures result in faster metabolism, which in turn mean the less oxygen is extracted faster and is needed in more quantities by the fish and other life forms than in colder temperatures.

A shortage of oxygen will be noticed by observing the Koi collection first thing in the morning. If the Koi are moving lethargically and hovering near the surface you may well have an oxygen deficiency. Oxygen shortages have been measured in ponds with fancy pumps and filters – but with a lack of exposing the water molecules to the atmosphere. A shortage of oxygen can also be observed by watching the breathing of the fish. If the fish are breathing heavily or “piping” they could be 1. Stressed or 2. Have a gill problem or 3. The pond water could be low in oxygen or 4. The fish could be exhausted for some reason (and what have you been doing during the night to exhaust yourself, my little Cynthia Sanke?).


Apparently, the larger fish die first, which would explain why the goldfish and the minnows are still alive. Miss Sadie, our goldfish, was basically resuscitated. She was piping/gasping for breath at the surface of the water. After we turned the waterfall/pump back on, she literally dove through the bubbles, seemingly having a grand old time!

I hope you learned something during today’s lesson.

  1. When in Florida, always assume there’s not enough oxygen in your outdoor pond.
  2. Plants consume oxygen at night time.
  3. Gasping for breath at the surface of the pond does not mean that the fish are looking for food!

Until next time…