I used to do that for a living. I don't know how it would translate into today's dollars, as this was back in the early 80's, but we used to charge $1 per gallon of tank size, per month. That included water changes, glass and gravel cleaning, and basically nothing else. Saltwater was 1.50/per gal.
That was for 1 visit per month. Additional visits were extra, and charged according to the urgency of the situation. You don't want someone calling you at 7pm on a Friday night unless you can make it worth your while, for example.
Critters were extra as well, but we discounted them substantially to customers w/ maintenance contracts. If you don't have your own shop, however, that's a difficult proposition as the mark-up on livestock is typically quite high. We did not provide any additional guarantees of livestock beyond what, say, SWF does.
As has been stated, you should also check into some sort of what's called "Errors and Omissions" insurance. On the level of business you're proposing, it shouldn't be horribly expensive, however. The likelyhood of ruining some expensive medical equipment is slim as the aquarium is more likely to be located in the waiting room than next to the MRI machine, but check w/ your insurance agent nonetheless.
I would avoid putting the policy in your parents' name if at all possible. You don't want their house on the line if it comes to that.
I'd also have a generic contract drawn up which indemnifies you in the case of some freak occurence, e.g. say the tank cracks or crashes under your watch - who's responsible? The legal language ought not to change from situation to situation, so it shouldn't be expensive to draw it up, but a definite consideration.
Equipment required would be what you probably already have, except you'll prolly need more buckets for replacement water, and something along the lines of a "Python" with a 50' or better hose. You'd have to factor the cost of water tests, if you intend to provide them as part of the service, into your cost of doing business. Don't forget to mark it up. Your time and expertise are worth money.
HTH.
