Water changes are simply the removal of some old water from your koi pond, and the replacement of that old pond water with new pond water. It sounds so simple but there are problems, nationwide. First, tap water can be chlorinated. Second, alot of people don't do pond water changes, at all. Thirdly, failure to do pond water changes allows the accumulation of background pollution such as phosphates and proteins which inhibit koi and pond fish health and growth. Finally, water changes need to replenish trace elements and minerals in the water which koi fish need.

Water Changes

Chlorinated and chloraminated water is usually supplied to hobbyists "at the tap" from municipal water supplies. The water company adds these two chemicals to disinfect the water. Each day, municipal source-water is tested for eggs, spores, ova and cysts of various pathogens. If any are found, it may be that the municipal water authority will double or triple the chlorine or chloramine concentration. Spritzing the water into the koi pond slowly WILL dissipate a lot of chlorine, but will it dissipate all of it? Dechlorinate. By dechlorinating the water, you can be 100% sure the chlorine is gone and will not harm your fish. When your municipal water supply uses Chloramine, you will be relieved to know that dechlorinator can still bind the harmful Chlorine. The remaining Ammonia should be no match for a cycled (properly functioning, well colonized) filtration system.

Change 10% of the water every week or...

Change 20% of the water every 2 weeks or...

If you're like me, change 30% of the water every 3 weeks.

In speaking to people from across the country, I found that about forty percent of the hobby is not doing ANY koi pond water changes at all. This accounts for recurring illness among koi and pond fish, slow growth, and poor color. This is the most common cause of the "seven inch, seven year old" Koi. A koi in good water with plenty of water changes should grow at least 3-4 inches per year.

"Topping Off" the koi pond is not a water change. You should know this about water: The solids in water do NOT evaporate, nor do many of the chemicals in the water. This means that the nitrates, phosphates, a good bit of the carbon dioxide, all the salt, minerals, etc NEVER leave the pond and accumulate over time. As the pond water level goes down by evaporation, you may notice that the water garden pond fish perk up as you add water back. There is a transient increase in water quality after the addition of 'new" water but it's rapidly offset by the dissolution of the existing background pollution. So, "topping off" actually concentrates solids and organic chemicals in the pond water over time. Real water changes should be endeavored.

Supplemental Reading:
Tap Water Can Cause Pond Fish Lose!