This site has a chart that tells you flow capacity of PVC
pipes. According to this 1" sch 40 PVC
will drain up to 960 gph
with gravity. I really wouldn't worry about the 18 being too much bc you will have at least 300 gph
of head loss, more with any elbows, unions, or ball valves. One other thing you need to account for is overflow box linear coverage as grizz previously posted. Also if the 18 is too much I would still use it just add a t onto the return line w a ball valve going back to the dump in area. I hate low flow I have a 75 with a 7 on it and wish I did at least a 9.5 maybe even a 12.