Always use good electrolyte, for flushing you just need to dump original electrolyte in a safe place and fill it with water multiple times to drain again. In last you need good electrolyte to replace original.
If your car batteries creates problems then you also need to verify your charging device which is alternator if that is working fine and charging the batteries to a proper value or maintaining a proper voltage on battery terminals or not.
I remember, battery on my corolla was not performing up to mark, I was about to change the battery when I detected a problem with alternator. After having replaced the faulty diodes, voltage on battery terminals was not upto the mark as it always stayed around 13.6 Volts. I was able to increase this value to 14.2 by installing a diode on reference wire of alternator and fooling the alternator to think voltage on battery terminal is less then what it is detecting.
Reference wire provides feedback to Automatic Voltage regulator such that If voltage on battery is as per manufacturer aspect or not. If not then it increases the output of alternator to compensate that.