Well, I don't actually practice what I preach in that respect neccessarily

I'm suggesting it to him as a means to conserve the next oil: water, because I am an environmentalist and think what he is doing is wasteful and unneccessary. Veiled agenda, yes, but also good fishkeeping. Also, while 50% 3x weekly might not be bad for the fish at all, it may not be any better for them than 50% 1x a week.
My tanks fluctuate with stocking too often for me to ever be able to have a fixed pattern. I breed a lot of different species and do my maintenance to suit the situation. I wouldn't be able to live with myself if I knew I was doing more water changes than I needed to for optimal fish health and thus contributing to the demise of our planet. Granted, one might say that if I feel that way I should get out of the fish hobby period, but I justify it by breeding some species that are endangered or extinct in the wild

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Discus wouldn't be a good basis for comparison either, since the average hobbyist isn't keeping discus because of their delicate nature. Also conditioning for breeding is good for your fish but it is also designed to maximize yield and induce spawning behavior. For many fish abundance of certain types of food and rain / flood water is a spawning trigger and so conditioning for spawning is also simply providing the conditions under which the fish is known to spawn.
I'm not saying any of this to be argumentative, it's again, just an attempt to conserve precious water while also looking out for the health of the fish.