Get yourself a 20G (or whatever size you want) storage container from Target or Walmart. Put it in front of the tank and mix your water there using the "python" to fill in the fresh water (I'm assuming it's not RODI then). Used measured amounts the first time so you can mark a line on the container and know how much salt mix to use.
If I'm doing a
water change, I'll set it up at night, let it run with a pump and a heater all night, measure the temp and S.G. to make sure they're the same as the tank, then drain the tank water from the sump (gets up detrius). I use the pump and pump the new water back into the tank, causing the overflow box to work its magic, and it's done.