In the United States, "Schedule" refers to the thickness of the pipe wall and, therefore, how much pressure it will hold. In most aquaculture applications, Schedule 40 is used because:
1. It works for most applications and is readily available.
2. In small quantities, it is only slightly more expensive than the thinner wall Schedule 120.
3. The wall is thick enough so that it will not distort when walked on.
4. From 1/2" to 2", it is rated at no less than 140 psi at 73°F. The highest pressure typically found at any facility is 65 psi, which is the average city water supply.
Schedule 80 is rated at no less than 200 psi at 73°F (up to 2") and is usually gray (similar to the Schedule 40 pipe used for electrical conduit).
There are very few reasons to ever need such a heavy, costly pipe for aquaculture. The outside diameter (O.D.) is the same on Schedule 120, 40 and 80, so an expensive Schedule 80 fitting could be used on Schedule 120 or 40 pipe, if that's all that was available. As the wall of the pipe gets thicker, the inside diameter (I.D.) gets smaller. With fittings, the outside diameter gets larger.
The main reason to use Schedule 80 is if it is mandatory in your area for specific applications but it can also be used on cold water loops to help insulate. <source