I was of the opinion for my tank that water changes were not as necessary as they really were when I got some more sensitive fish, Dwarf CPO crayfish and otocinclus catfish. Along with my 2 dwarf gouramis and 18 neon tetras in a 20 gal long tank. I have it decked out with plants so the parameters were always pristine, for months I only topped it off and the fish were super happy. After a while the ph dropped to much and the crayfish died, and then one day I decided to do a regular 30% water change to get built poop out and after that I ph shocked my Otos and they died. I think if I just did weekly water changes at 20-25% I could have kept them all successfully, but I also got really depressed and "didnt feel it was necessary". Now the only ones left of the original stocking are my dwarf gouramis and neons, seeing as they are much hardier fish. So dont overstock your tank, and dont let the ph fluctuate too much, regular water changes are the best way to control ph. So in my opinion, yes, they are necessary.