The best idea for this Warr, if you had space, would be to do the following:
- have a seperate 100 (or bigger) tank plumbed in directly with your tank (or perhaps even 2 tanks, of equal size, would be even better).
Then, mix your fresh salt water in the one tank, have all taps/valves in the plumbing closed at this stage. Have 2 pipes going in and out of both the tanks.
Then, open the tap that leads into the empty tank (old tank water). Then once you have emptied enough into the "empty tank", you can close the tap/valve, and then open the valve/tap coming from the tank with the "clean water". This tank has to be high than your DT.... the fresh water will then run into your tank.