I'm a lurker with a 29 gallon as my "big tank," but thought I'd chime in on this. I have pretty severe depression and have issues motivating myself to do even basic stuff like get out of bed or eat. My fish are definitely keeping me sane-ish. Knowing something living is depending on me motivates me even in my deepest funks, and cleaning the tank and doing water changes is my zen. On my worst days, I can easily bring myself to do something (even something as simple as scrubbing the glass), and that gets me moving so that I can try to tackle other life stuff. Even if nothing else gets done during the week, I get my water changes in and watch the fish eat twice a day.