I don't find large water changes to be a problem. One of my tanks routinely gets a 50-70% weekly water change and just this past couple of weeks, due to remodeling, moving tanks around, etc. just about all my tanks have gotten two or three 80% water changes within just a day or two. Filtration and fish didn't skip a beat; in fact, they look great since as long as I was going that far I also rinsed just about all my filter media at the same time (non-chlorinated water). I won't speculate or try to explain why some people have issues with large water changes. And, in fact, every so often I like to do successive water changes within just a couple of days that replace most, all, or even more of a tank's total water volume as a sort of refresh/tonic for the tank.
But you'll always get different answers on this. So it's is a subject where you should do what works best for you. There's just way too many variables involved for there to be a one size fits all formula, including: size of tank, number of fish, size of fish, species of fish, age of fish, type of substrate, depth of substrate, type and volume of filtration, type of media, how often you clean (or replace) your media, plants, no plants, number of plants, species of plants, areas (like driftwood) for bio-film to grow, whether you even allow algae or other bio-film in your tank or keep it spotless (whether using plecos, etc.), what you feed, how much, how often, water temperature, pH, dissolved oxygen in your tank, even whether you run UV or not (in combo with filtration and other factors, can affect redox and, therefore, breakdown of wastes)... quality and chemistry of your replacement water.
I don't even treat my each of my own tanks the same, because even with the same water and basically the same food for all my tanks, different setups are different in some of the other factors, though someone else might prefer to do the same % water change across the board on every tank-- but I do or have done anything ranging from 30% weekly, 70% weekly, 10% weekly, 50% monthly, and 60-70% bi-weekly on different tanks. I've slowed down some the past couple of years, but over the years I've spawned and raised thousands of fry, raised I couldn't tell you how many species, and have had stretches of five years or more without a sick fish, so I must be doing something right, not that this hobby won't throw
anyone a curve, no matter how expert. But there are a
lot of very skilled and knowledgeable hobbyists around, some of them way ahead of me, so I'm not trying to brag-- just making the point that there's too many variables to make a one size fits all recommendation.
Finally, from a post of mine on another forum:
I've read different and contradictory theories on water changes and nitrates, including one that created the impression that nitrates basically increase infinitely, despite water changes. His theory was that each water change reduced nitrates by a percentage, lets say 50% with a 50% water change, but since your fish are always producing a fixed total of nitrates the effect is something like this: fish produce 10 nitrates (just a theoretical number for the total they are producing) and you do a 50% water change, which leaves 5 nitrates. During the following week they produce 10 nitrates-- total in your tank is now 15 nitrates because of the 5 left over from the previous water change-- so now change 50% of your water and you have 7.5 nitrates. The following week your fish produce 10 nitrates, making your total 17 nitrates. Change 50% of your water and you now have 8.5 nitrates, etc.
The article left the impression this increase goes on indefinitely. Nonsense. If that were true nearly all tanks would have astronomical nitrates, except for the few who are constantly changing all or most of their water. And the fact is my tanks are low, for example my Kapampa tank normally reads about 5, with typically 30-40% weekly water changes. Given sufficient water replacement, at some point you should reach an equilibrium between the nitrates you remove each week with water changes and the net your tank is creating each week-- this assumes nitrates out of your tap are zero or very low. Of course, if your tank produces a lot of nitrates and water changes are very low, nitrates can get very high, but infinitely high?
Sorry for the long post, hope it's helpful...
( lol persistence I have tho