Every morning when I go out to the fishroom, I get washed over by a wave of contentment. That's not a good choice or words, but as close as I can get. I step into that crowded, noisy room, and while I'm checking the progress of things, how equipment is working, what the fish are doing - I just feel good.
Sometimes I have tasks I may not be as into at that moment- maybe water changing, filter cleaning, etc, but they always somehow lead back to enjoyment. I guess people with gardens, religions, running or workout schedules, a love of cooking - anything like that would understand. I used to get that feeling from my job on many days - a feeling free of excitement but rich in peace and engagement.
For me, it isn't one tank that does it - I don't have a favourite. But when this hobby works for a person, it works very well.