I've done the same thing, maintaining tanks of a few customers, of my LFS. I billed monthly, based on how much work was to be done, on each tank. For instance, here's 2 examples, extremely different from one another:
#1. 150 gallon reef tank, the owner of the tank didn't want to have to touch anything, ever...lol. He wanted a beautiful tank, to show off to his friends, but didn't want to touch anything or learn anything about it. He reminded me of a rapper wannabe! He demanded weekly maintenance. It ended up being much more than "maintenance," as he also wanted his substrate completely removed, replacing it with black sand. He had a RO/DI unit, hooked to a holding tank with an auto float switch. Our contract stated that he'd have saltwater mixed, when I arrived, and it would be at the correct salinity. That NEVER happened...lol. I'd show up, to do a large water change, to find cold RO/DI water. It was infuriating! He also shopped at another LFS, who was only interested in making money. Each week, I'd show up to yet another fish or 2 in his tank, rarely were any compatible with one another and sometimes they weren't reef safe...lol. I charged him $400.00/month. I finally told him that if he didn't stop adding things to his tank, based on the second LFS "advice," he'd need to find another maintenance person. The last straw came, when I arrived to find he'd added a shark to the tank!! I quit...lol.
#2. 200 gallon reef tank. The owner was very interested in his tank, but worked out of town, leaving him little time for maintenance. His tank was in BAD shape, when I started. He'd never used RO/DI water and the tank was an algae swamp! However, he was very interested in learning and helping fix the problem. This guy is LOADED and willing to pay any amount. He offered me $500.00 per month, to maintain his tank. I refused to take that much, as I felt it was way too much. For the first few months, WE did weekly water changes, with RO/DI water. After we got his tank cleaned up and stable, we went to monthly 50 gallon water changes and regular testing/dosing of Ca/Alk/Mg. Eventually, a time came when I only maintained his tank when he was out of town. Otherwise, he learned to enjoy doing it himself. I charged him $250.00/month, for the first 2 months, then $150.00/month after that. Now, he throws me $50.00 or so, every time I do maintenance, when he's out of town. He also knows that he can call me, in the middle of the night, if an emergency crops up, which he has, twice. Once a snail plugged up his overflow drain, causing the tank to flood...lol. Another time, his return pump stopped working. Both times, I rushed over, in the middle of the night, fixed the problem, he tried to pay me a couple hundred each time, I refused. We've became great friends.
Basically, I think a general rule of thumb is $1.00-$2.00/gallon, depending on how labor intensive the job is. If you have to transport water, the price goes up!