Look at the AC voltage regulation section of http://en.wikipedia.org/wiki/Voltage_regulator#AC_voltage_stabilizers, particularly the ferroresonant transformers. I would use one to power everything else and let the pump sag. Sola is a common brand. New ones aren't cheap, but there are a lot left over from the days of big iron computers.
Look at the AC voltage regulation section of http://en.wikipedia.org/wiki/Voltage_regulator#AC_voltage_stabilizers, particularly the ferroresonant transformers. I would use one to power everything else and let the pump sag. Sola is a common brand. New ones aren't cheap, but there are a lot left over from the days of big iron computers.