So what application requires sch 80 pvc and what is ok for regular?

Schedule 80 pipe and fittings are designed for higher pressure applications and will have thicker walls. That being said schedule 40 is rated to much higher pressures than we will be dealing with in our regular aquarium plumming, so aside from physical abuse schedule 40 will work just fine. However if you can only find the schedule 80 version of a fitting you want, no harm in mixing that with the schedule 40 pipe etc.

Here's some further info if you like:
http://pvcfittingsdirect.com/_e/page...chedule_80.htm