Is that a heater in there. You can take that out since horse tanks need to be 74 degrees or lower.
What size and brand of powerhead is that? be sure to cover it in some way to prevent little tails getting caught.
Is there anyway to attach a spray bar to the back where the return is ?? that would give you a nice gental flow.
For hitches, you will need plants, gorgoians or something fake, like fake corals or plants. You will need them in several places for the horses to hide and or hitch. They have to be able to get thier tails around it.
Oh ya, where do you live?? you can always get two females and not have to worry about babies until you are comfortable and decide its time to upgrade. heheheheeeee
T