Guppies are your best bet if you want to turn any kind of profit at all in a single tank breeding operation. Beyond that you might try yellow labs. Truth be told though just because your dad pays doesn't mean it costs nothing.. that's your inheritance he'll be spending! More than that, as mentioned before, with just one tank you will never really be able to make any serious money. I'd strongly reccomend you go with guppies.. not only are they the most likely to make you money, if you've never bred anything before you'll have a hard time being successful with any more advanced species. Not that mbuna are difficult to breed or anything but stripping mouthbrooders of their eggs can be difficult for the most experienced keepers at times.