These two fish are from the same brood and purchased at the same time so why are they so different in size? 

The bigger fish (about 11 inches) was kept in a 500 gallon pond with optimal water quality and was fed by a waterfall and a 25ft stream. It is 18 inches deep.

The smaller fish (about 6 inches) was kept in in a 2,400 gallon pond with similar water quality but no water features. It is 36 inches deep.

Both fish were put in their mature ponds that contained other fish in July and this picture was taken the following May.

All water quality tests were taken and compared and found to be within normal range. The oxygen level in smaller pond was 8 and in the larger it was 7.

  • So what led to the growth difference?

    With all the algae in the stream being pushed into the pond there was more availability of food early on.

    With the water being shallower the ambient water temperature was higher as the weather warmed. Don't confuse this with shallow water is better. Shallow water has more temperature swings in the extreme cold and heat which can kill a fish. The deeper the water the more stable the water temperature.

    There was more of a current in the smaller pond, forcing the fish to exercise and possibly be healthier.


  • If you want the largest fish possible keep winter water temperatures above 65 degrees and summer temperatures below 85 degrees.

  • Feed your young fish, up to 8 years old, high protein food as the temperature warms. Feed them wheat germ from 45-60 degrees (Cheerios is a good source). Do not feed them when the temperature is below 45 degrees.

  • Create variability in current and surroundings so fish will ways to stay healthy.

  • Of course, as you learn more about your pond you will see other ways to help them grow healthy.

Frequently asked questions

  • How fast do fish grow?
    Koi grow similar to humans. They grow more in their younger years than in their adulthood. A 2-3 inch koi is a year old. A 6-8 inch koi is 2 years old. Goldfish have a similar growth pattern (see chart below). How fast, and larger, a fish grows is governed by the fish to water ratio being correct, the water temperature year round (fish grow larger when temperatures are kept above 65 degrees during the winter), amount of protein in food (fish fed 55% protein will grow larger compared to the common 33% protien levels)  and the water quality.

  • Is it true a fish will only grow as big as the pond it is in? 
    Fish are no different that any other animal. If you put a koi in a small poor quality pond and it's twin in a perfect sized and healthy the pond the fish grown in perfect environment will be twice the size of the other in as little as a year. It will also live longer and be healthier. Does that mean the other fish stay small because of the pond size? No, there are samples of koi being kept in a 20 gallon tank that fill it from one end to the extent it cannot move around in the tank. Yes, it did not get as big as it could have but it is also stunted in its growth. Would you live if you were kept in a 6ft x 6ft room - yes but why would you you want to.