To try and bring this thread back to it's original intention and to let Gilpi know that he/she took my comment out of context, the thing you all seem to be forgetting is that ( and I'm old school here so bare with me
), An aquarium is supposed to be an ecologically sound environment in which the aquatic life inside the aquarium can not only live but thrive. It's not a box in which we try to stuff as much as you can into it. In order to keep this equilibrium, there needs to be things done to the tank because it's a closed system. Water changes help: purify the water, replenish minerals and trace elements that the fish absorb and remove from the water as well as remove and dilute the last step of the nitrification process which gives us nitrates.
This is not all that it does but these are the basics. If equilibrium wasn't an issue, there would be no reason to cycle an aquarium, just change the water. Don;t filter, just change the water. Don;t plant the tank as a means to remove nitrogen, just change the water. Get my point?
As for how much, it's been discussed already that there is no "average" tank so there can't be an "average" amount to change. The comment my Mentor used was to help guide me to better husbandry of the fish I kept and should be a guideline to other fish keepers as well. Fish are not always forgiving and many a tankful of fish have been lost due to improper husbandry. My mentor's suggestion was that by doing certain things routinely but not extremely, I should have a better success rate just as he did for all his years in the hobby. ( He started with fish back in the 1930s. ) This is my 50th year keeping fish and have had so much success in that time doing the things I was taught in the beginning and still do today. In today's "hobby", extremism seems to be the norm. Is it necessary, really? The important part of my comment that Gilpi brought out is that "if your fish look better after a water change you didn't change it soon enough." What that translates to is that your tank is not being managed properly and your fish are suffering while waiting for you to clean up after them. My fish look just the same after a water change as they do before it because I don;t let the tank get to any extreme levels. That's been an invaluable system as it's helped me breed many types of fish ( most of them wild caught or newly domesticated.) So the system shows that it works.
As for the tank being a "toilet bowl", if this is true, then you also have to consider any lake, pond, stream or ocean that has life in it as one as well. HOWEVER, just as these bodies of water have ways of cleansing the water of these "impurities", our tanks have also in the form of filters and absorbent materials we put into them. So if your tank is a "toilet bowl", shame on you for not taking better care of your pets.
For any newbie just starting out, how are they to know what is "right or wrong" if we don;t take these threads seriously? I'm all for a good joke but this is no joking matter. Doing this wrong can lead to a tank full of sick fish. Just look at the thread numbers in the "sick fish/ unhealthy fish" forums. Doing this wrong can kill a tank full of fish as well. So should we really be joking on this subject? So since this is the internet and anyone with a keyboard can post anything they want with no repercussions, to those who need help, feel free to PM me with any serious questions about fish keeping. I will do my best to help you. For those looking to fill my inbox with nonsense, be aware that I will forward your PMs to the administrators. DON'T waste my time.
Hope this helps