I actually did an experiment....
Now guys, you may flame me, riot at my house, and take away my sisters/parents...But please don't take away my fish.
So I got a 125 gallon tank almost two years ago. I had 6 or so LITTLE fish...Maybe 3 inches each, just assorted species. Now the average length grew to about 5" in probably about 8 months. I did do water changes in those months regularly.
However, I decided I would try an experiment. So for about a year....I didn't do any water changes. My filtration was actually really inadequate as well. It was like a whisper 30-60 or whatever haha. So basically, in that year...I lost no fish whatsoever. Not a single one died, and I actually stocked the tank from 6 fish, to about 35. Most of these were neon tetras, serpae tetras etc. Small fish, you know? Not much of a waste producer, but still. 35 fish. 125g tank. Bad filter and no water changes. Finally, I did my first water change in almost a year, and did about 70%. Now during the time I didn't do water changes, the water was fine. It looked fine, smelled of nothing, and all the checks came back fine. And remember, not a single fish died in this year. Now about a week ago, I decided to strip down the tank and stop my experiment. So I did. Replaced all the gravel with sand, and added some gravel on top of half the sand, and I replaced my crap filter with an fx5. I replaced all the water as well...Now I put my 20 or so fish back in, and the water is great. The fish are great, and I still have my original fish that I bought at petco 4 years ago.
Just a little test I did to see if water changes were absolutely necessary. I concluded that they are, in fact...Not. But, they are VERY useful. The only time you don't want to do a water change, is if you have good filtration, one neon tetra, and a 500g tank.
Ok, maybe a bit of exaggeration, but you know what I mean. Water changes for me, for that stock, and my tank...Were not necessary in a year to keep the water in check. Now, though, I am going to be keeping up with water changes twice a month.