what filter should i get if i’m planning to put turtles and small fish like mollies minnows and gouramis??

It's not selected according to fish type. It's selected according to volume of water so you would buy one that meets or exceeds this 125 gallon capacity. On another note, however, you probably do not want to stock it with tropical fish as they are not suited for outdoor climate (depending on where you live — although Hawaii might be the exception). Most outdoor ponds are stocked with carp (koi) because they can tolerate cold water temperatures, unlike tropical fish that would require a heated pond if situated outdoors.