Since I have a large trash can to circulate for my water changes (40 gallon), I have one powerhead on the bottom to ensure that the salt is not sitting and accumulating on the bottom, and I have one on top making sure that the water is well oxygenated. Dissolving salt with water uses up a lot of oxygen, and it is very essential to put this O2 back in the water. In order to do this, strong surface agitation is neccessary.