If you would not swim in your pond,
your fish shouldn't be either.
Water Changes Are Critically Important
Water changes are simply the removal of some old water, and the replacement of that old water with new water. It sounds so simple but there are problems, nationwide. First, water can be chlorinated. Second, a lot of people don't do water changes, at all. Thirdly, failure to do water changes allows the accumulation of background pollution such as phosphates and proteins which inhibit fish health and growth. Finally, water changes need to replenish trace elements and minerals in the water which fish need.
Chlorinated 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 pond slowly WILL dissipate a lot of chlorine, but will it dissipate all of it?
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.
In speaking to people from across the country, I found that about forty percent of the hobbyist are not doing ANY water changes at all. This accounts for recurring illness among the 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. Hobbyists should be encouraged to follow a water change regimen as outlined in the chart below.
"Topping Off" the 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 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 water over time. Real water changes should be done.
Ideal water change regimens
Every week: 10 percent water change
Every two weeks: 20 percent water change
Every three weeks: 30 percent water change
No matter which of the above regimens you pick from above, I HIGHLY recommend that twice to three times per year you should perform a 60-70% water change to really REFRESH the pond. You will notice a real boost to fish health and growth. No matter which of the above regimens you pick from above,
Major water change: Simply drain the pond down 60-70% and add dechlorinator. Then refill the pond. Don't do this in the PEAK of the summer as you might chill the fish (I've never hesitated, but that's just me I slowly add cold water to my ponds My KOI love to play in the shower). But SURELY in the early summer and late summer you should find the fish VERY appreciative of this service.
If you are performing the recommended water changes, you should have robust, hungry, healthy, happy fish. Fish may still become ill, of course, however it is less common in well managed ponds with LOTS of FRESH Water.