So most of the responses I had come up with has already been covered by the past 3 pages of responses... just wanted to add another post suggesting the same things.
Here's one idea that hasn't been mentioned... and is a semi-cruel way of getting the point across. Keep the kitchen trash can in his bedroom. (assuming he has his own room... if not, this idea won't work) Tell him he can only take out the trash once a month. See just how quickly he agrees that waiting a month for it to build up is far too long, and compare that to fish waste building up for a month. If he argues its not the same, point out that you're not even making him gather all of his "waste" to decompose in his living environment, and that he needs to just drop it.
Weekly or more often water changes are much healthier for our fish than letting it go for a whole month. I'm personally really mad at myself for having let my tank go almost 2 weeks without a water change... I usually do at least 2 50% changes a week. (the fact that I've been crazy busy of late doesn't make up for it, but it is legitimately true)