With saltwater I'd suggest you work for another aquarium maintenance company for awhile. This would even help with freshwater maintenance.
Doing maintenance for other people isn't just like on your own tanks. You need to have equipment for all size tanks, and also for ponds. You need to realize that you may be working on tanks that have had poor maintenance before and are horribly overgrown with algae, bad water quality, etc. Some customers are going to have requests that conflict with your personal practices. And you need to know about or have the ability to quickly research various species and tank setups to know their specific needs so you don't kill the stock or mess up equipment (for instance say you don't know much about rays. with most fish you add coppersafe. you do the same with these rays not knowing better. OOPS).
You need to know what stock of items you'll need, including a fair amount of replacement equipment in a pinch, or know where to get it quickly, for people who call you in because their filtration failed.
And that's just a few things. I'm just starting up my own business which will include some maintenance services, but I'm friends with a few fish store owners that also offer maintenane services and I know what kind of equipment, time, and service is needed.
You also need to have great customer service skills since you will be competing with probably several other businesses in your area (most fish/pet stores offer some level of maintenance service), and your prices are going to have to be competitive from the start, which means you're likely going to lose money for awhile, between you taking longer to get things done than those experienced with that business, and initial startup cost for equipment, licensing, etc., and you'll want to plan to offer at least some services that aren't regularly offered by others so that you have something different about your service that makes people want to use you, and allows you less competition from other businesses.
For instance some businesses tout prices, some great customer service or 24/7 assistance, some specialize in specific set ups. You need to know and find a niche before you start.
