A healthy pond should be relatively bubble free. If you have some bubbles forming on the surface of your pond it is usually caused by a build up of dissolved organic compounds. This build up of organics in the pond can be caused by several things. One of these reasons may be that you are over feeding the ﬁsh. On the other hand, you may be feeding them just the right amount of food, but if it is early in the spring and the beneﬁcial bacteria in your ﬁlter has not had a chance to recover from the cold winter water, it will cause bubbles. Try cutting back on the food for a while to see if the ﬁlter is able to catch up. If you are still getting bubbles during the middle of the summer your ﬁlter may be too small to handle your current ﬁsh load. Consider either reducing the number of ﬁsh in your pond or moving up to a larger ﬁlter.
If suddenly, over a one or two day period, you get a lot of bubbles on your pond surface it may be that your koi are spawning. Look carefully at the sides of the pond. If they have spawned you will probably see hundreds if not thousands of very small eggs, about the size of the head of a pin, attached to the sides of the pond. If this is the case you will need to check the pond’s water quality over the next two weeks carefully (See test kits for koi ponds). When larger koi spawn they release a huge amount of organics into the pond and ﬁlter system. Most ﬁlter’s bacteria is not prepared to cope with this extra load. You may get a spike in the ammonia and nitrite levels. You would do well to clean your ﬁlter as soon as you can in order to remove as much of this organic load as possible. If you are hoping to get some new koi from this spawn, you will more than likely have enough eggs in the pond to have some survivors. If you don’t clean your ﬁlter, your water quality may degrade to the point of endangering all of your koi.