well when i replace some evaporated water, i need a ph boost 'cause my ro/di is at (ph) 7. It would slowly drop during the week, so i use seachem marine buffer 8.3...half a coffee spoon every 2-3 days, depending how much water evaporated. ( usually 1/2 to 3/4 gallon per day with my mh at 5" from the waterline ). Get a 10 % evaporation rate. Quite enormous.
BTW the new water isn't mixed with salt.