If you need to pull 1800 GPH, you should go with a couple of 1.5's this allows for failsafe if one clogs for some reason.
If you are pushing 1800gph at 3/4" line, it will be powerful enough to cut drywall. or stir the crap out of your tank. I suspect you will run nowhere near that with 3/4". what pump are you using? Most pumps 900 and up should be using 1.5" for their return line.
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