will this run a 125 fish only

This needs to be posted in the general forum please. I will move it for you.

 

Depends on what you want it to do? If it's just used as a return pump then it will be fine. I don't run much volume through my sump and prefer to use powerheads for circulation.

 

What size is your drainline? You want to make sure that your drainline and return work in unison so that you are not draining faster than returning and vice-sersa. You might need a ball valve on your drain but again it depends on the size of your drainline. Fish only don't need a large amount of flow in a tank (as when one has corals) but you do want sufficient turnover in the tank for ridding it of detritus. Will you be using LR?

 

Your drain can be obsenely over sized for the pump. Its really of no concern if the drain can handle 1200gp or 120000gph. In fact the more oversized the drain is the better this ensures you don't overwhelm the drain and have the display spill out over the floor. An overflow standpipes drain speed is dictated by the retun pump and the return pump only.
A siphon overflow would be the only instance the drain size would need to be adjusted, I've heard of fools attempting to do this and they fail everytime, usually within minutes. You cant use a siphon drain and equilibrate the drain and return speed, stand pipes are the only system that works. Flow rates of siphons and return pipes are influened by everything from barometric pressure to wate temp, viscosity, specif grav, and more.
In short, that pump should work fine for you 125, and you can use you powerheads to keep detritus in suspension. You may find that depending on how much rock you use you may want more powerheads to achieve adequate detritus removal

 

Unfortunately there are people who do use a syphon overflow and that's why I mentioned it...sure don't want a flood ;-0)