I worked at a local shop for 5 years before I went out and stared servicing tanks ponds on my own. We made most of our profit in freshwater livestock and dry goods. Marine and reef was always hit and miss. I remember one year I found a graph that showed how much in sales I made for the buisness on a month to month bases. Even if the owner only made 10% of that it would have paid for the power, water, and all employee wages (I knew how much we paid in pg&e and cal water I practical ran the buisness end half the week alone). That was just me, no other employee or the owner. And did not include the tank service side of the buisness.