Joe,
Sodium Percarbonate is cheaper than 30% hydrogen Peroxide. However my experience shows that sodium percarbonate only works like a miracle the 1st time. Repeated uses are less effective. I suspect it leaves behind something in the soil. Therefore I think follow up treatments, if any, should be with 30% Hydrogen Peroxide. Both products oxygenate the water. Be prepared for VIOLENT bubbling of the water in your pit. Protect your eyes & skin if the bubbling shoots out like a volcano! The reaction is amazing! Once the water level starts dropping then install an aerator to keep the water oxygenated. See this site for details: http://www.oxyseptic.com/
Call them to ask how many lbs of Sodium Percarbonate to use on your 1,500 gallon pit. They recommend 36 lbs for a septic field.