Sump tank heaters

[Kali155]

So I am looking to start setting up the sump for my tank. After doing some research for my 155 gallon tank it recommends 500w heaters. Talked to guy at lfs and he informed me that I can go with 300w. Sump is around 40 gallons. Should I go with the 300w or 500w? Heaters will be located in sump of course.

 

[JoeMan]

Id go 300w

 

[JoeMan]

But two of them

 

[Kali155]

Yeah that's what the lfs guy said just wanted to make sure that would be sufficient