I would actually do some research and find out how many other people are doing the same thing in your area. Ask at your local fish store. If there are too many people doing it, you may find it difficult to break into the market in your particular area unless you can make a name for your self.
You also need to know every aspect of aquarium maintenance. For instance, if somebody asked you to look after a Marine tank then you're going to need to know the ins and outs of the various components that people have on saltwater tanks, such as sumps, skimmers etc. You will also need to know how to check the salt levels etc. etc.
I think it's pointless starting a business if you can only carry out maintenance on tropical tanks, I would imagine there are a few people who keep Marine and that is probably where the money lies. Many people will probably want to set him up as well.
Difficult to say how much you can charge, you would have to find out what the going rate is in your area, or find out what people are willing to pay. Around here, a typical charge would be around £35 to maintain a tank of 100 gallons. That would be a water change, vac and filter clean.