Well, I'd swear I posted a response to this last night but I can't find it anywhere.
Anyway, as John said, add only water unless there is a spill. Only the water evaporates and needs replenishing. However, since you noticed the bubbling while running and the electrolyte level was low, it is very possible that your battery is over-charging. If you have a 3 brush generator, you can slow the charge rate by moving the third brush. Otherwise, your suspicions may be correct on the regulator. If you have a voltmeter, check to see what the voltage at the battery terminals is when the system is charging.