What are external pumps for?

[king_neptune]

I was wondering what kind of setups are external pumps used in?
I am building a 125DT with a 55gal sump. The drain off goes into the sump via gravity and the return is an in sump pump. Would I need to have an external pump in this system?

 

[videoguy]

external pumps are used to lower the exchange of heat into the water. I run a in-sump mag pump which will transfer heat into the water. I would recommend an external pump if your having issues with water overheating.

 

[spiderwoman]

We are using an external pump in our 150g system.
There are many reasons for using an external pump, one is if you don't want to waste room in your sump etc.
This is what our setup looked when everything was being setup. Now we have 2 external pumps, one for the chiller.

 

[oceansidefish]

I use an in sump mag 1262 for my 120....Have not had any heat problems with this pump