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Fish stocking density is affected by many different variables. Larger fish require a LOT more water than smaller fish. The number of plants in your pond, the type of filtration system, and the dissolved oxygen levels in your pond affect it. The waste handling capacity of your filter system and how much of what quality food is fed plays a big role.

In other words, that’s a tough question to give a definitive answer to. Suffice it to say that it’s likely to be a whole lot fewer fish than you initially planned on getting. Most small water gardens (under 500 gallon) should stick to goldfish and avoid the much larger koi. Remember that the little fish you buy will grow FAST in a pond environment, and if too numerous, they can outpace the capacity of the filter system and result in dead fish. Besides that, within a season or two you’ll start seeing baby goldfish who will ALSO grow to full size.

In general, each adult goldfish needs at least 15-25 gallons of water and each adult koi needs 100-300 gallons or even more. Unless you plan on substantially increasing pond size and/or filtration systems, you should plan for the eventual growth of the fish and keep the numbers down. Work closely with your local pond dealer to make sure you don’t overpopulate your pond. Having a few very healthy fish who grow to attain their full color and size potential should be desirable over a whole herd of stunted sickly fish.

Adding new fish to an Existing Pond Population

Great care should be taken when purchasing a new addition to the pond. While koi and goldfish are rugged and hardy creatures, they suffer tremendously from the stress of transport to a retail facility. They usually arrive at your dealer with their immune system suppressed from the strain of transport and can be harboring deadly disease and/or parasite problems. These are things that you DON’T want to have to deal with in your pond. In particular, avoid the big super-chain stores that occasionally get large batches of fish in for cheap prices. Take note of the above info on overcrowding and picture the poor transport-stressed fish crammed 30 to a 20 gallon tank just like the tropicals………. then look closely for the ones that float or lay quite still on the gravel, they’re dead, you know!

Either buy fish from a reputable dealer who quarantines his newly arrived fish, OR, build yourself a quarantine facility and keep the new guys isolated for at least 3-4 weeks BEFORE adding them to your pond.

Some folks like to add fish of other species to their goldfish and koi ponds. Pleocostumus, or algae eater or sucker fish, are usually one of the first choices. This is usually not a good idea for several reasons. All of those other fish are tropical, and have different water temperature requirements than cold water koi and goldfish. A Pleco may do fine all summer, but it’ll die if left in the pond over winter. Besides, the only “algae” they eat is the beneficial carpet algae that grows on the bottoms and sides of the pond.

Add Only a Few Fish At A Time

Don’t add more than 1 to 3 fish to your pond at a time. Adding too many fish at once will produce more waste than the filter can convert. Allow about 10 days for the biofilter to increase it’s capacity and absorb the increased waste in the pond before adding another 1 to 3 fish.