When I took apart my watermaker last week I saw no way to tighten the belts. So over tightening the belts seems unlikely unless you have replace them with shorter ones.
It seems like your pump is taking more power than expected to rotate. Did something change about the pump?
I did replace the oil in mine (using standard 15W40 motor oil as prescribed). There is a small indicator that tells you about the oil level.
Did you take any of the single direction pulleys off? Did you put them back on in the correct way?
Also you may have introduced a blockage in the path after the pump. Does it run smoothly when you take off the HP hose from the mebranes?
Just some suggestions to help determine what is wrong…