While being on the koi/goldfish show a couple of weeks ago, I listened in on an interesting discussion about the importance of waterchanges, and this is what I learned....
Participants were a half a dozen experienced koi keepers with a few expensive koi in their collection (except me, of course ), and they were trying to explain a newbie why it is so important to change the water frequently...
Its not only to reduce the amount of waste and nitrates in the water. More importantly it is to get something out of the tank that would hinder the growth of the fish....A fish expells a growth inhibitor while breathing, and that growth inhibitor remains in the tank/pond, and with enough accumulation due to lack of water changes it hinders the fish in his growth, meaning the fish will stop growing. With water changes the inhibitor will be deluted and reduced in the fishies environment, and the fish can continue to grow.
As an example one koi keeper told us about two specific koi ponds in his neighborhood. Both ponds had about the same volume of water, with more or less the same amount and sizes koi in there, and since they were in the same neighborhood, the same amount of life foods available to them like mosquito larvae and daphnia.
One pond had a regular filter system with it, the ones we would have in a smaller version on our tanks - mechanical and biological filtration. Water changes were done on a regular basis once a week, always the same amount of water.
The other pond also had a filter connected, but it had a continuous system where the water would be drawn from a well, conditioned and pumped into the pond, while the overflow constantly was putting water back into the environment. This pond had constantly fresh water coming for the koi.
Both koi keepers fed their fish the same food (HiFeng growth food for koi), but the koi in the pond with the constant supply of fresh water had much bigger koi in there in a much shorter period of time. The growth inhibitor in that pond is constantly being swept out and away from the fish.
NOw, you might say, these owners bought fish from different sources and thats one explanation on the different growth rate. But they took it further, they both bought a dozen goldfish from a local koi importer, from the same breeder in China, and each of the two put his fish into a separate pond on their properties. Same thing happened again - the goldfish in the pond with the constant fresh water grew bigger faster than the ones in the pond with the weekly water changes.
This does not mean we should perform daily water changes in our tanks, but it means one thing - keep up the routine water changes, don't be lax in it, do a little extra for your fish, and one more thing..... Food and more feedings are not the only thing that you need to know about if you wonna have big fish. The growth rate of fish is a complex story, and involves a few other things than just putting exsessive amounts of food into our tanks and hope for the best....
So, lets roll out those buckets and containers, and the calendar, so we don't miss any of those very important water changes!