You have to think about alot of things when taking over.
1. location vs measures of advertisement taken in order to achieve what you have set forth as realistic profit to debt ratio..... how much you spend vs how you make off your supplies and livestock.
2. Vendor relationships, associate yourself with vendors ie. suppliers that carry quality animals at attractive prices. You dont want to spend a ton at your wholeseller because then you have to jack up the prices on the retail side. This will drive away clientele. Same goes with the products you sell, food, tanks, etc etc
3. Promotional events, have fish swaps, hand out fliers, advertise in the newspaper put car magnets on your car. This along with supplies and livestock will be your biggest costs, if rent and utilities arent that bad in your area. (remember you want word of mouth to be the best tool for adverstising, its cliche but it works!)
4. GET A GOOD ACCOUNTANT, or a good accounting program so you have all your debts to income separated. This will give you a good indicator on how much you make vs losses, liabilities.
5. Fill your brain with as much knowledge of products and live stock as much as you can. If you dont have an answer keep books around to get the answer.
Good luck!