Adding water is a "daily" activity. And the amount depends on the heat of the day influenced by inside temp, outside temp and how much light you have on the tank. Set a mark where water should be and restore it to that line daily.
The only time we use salt water is for a water change and MINOR adjustments can be made at that time to restore your ideal.
Also watch water that you take out of the tap. If this is the water you are using keep a bucket filled to let chlorine evaporate and top off the tank from the bucket and then top off the bucket for futures.
If you are already in "salinity trouble" and things are still alive then help your evaporation out a little bit - in addition to what has evaporated dip out an additional large glass of salty water daily and replace with fresh.
Continue this additional out with the salted and in with the fresh until you have your salinity correct.
This way you can come back in line without making a sudden change.
yeah, the RO/DI unit I bought has been one of the top 3 investments of my tank. it's nice having control over the water you put into your tank. you never know what type of water you are getting when you buy it from the grocery store.
as far as high salinity i'd just do a gallon or so of water change every day until you get it down to about 1.025 or 1.026 depending on where you want it. i keep mine at 1.026.
fish can handle higher salinity but inverts are very intolerant to high salinity.
Think of it more in terms of a water treatment system, it does so much more than one of them screw on the tap and have mountain fresh water thingies
Tap water is forced into the unit using line pressure, generally thru a sediment filter that traps large dirt and mud particles, rust flakes and other nasties.
Then it goes thru a carbon filter that removes, chlorine,chloramine, voc's and other nasty chemical compounds.
Then its forced against a permeable membrane that will allow water and not much else to pass thru, so that a lot of stuff that is measurable as dissolved solids (like the calcium that leaves water stains behind) is diverted out the side as waste water. Thats why you get a usable to waste water ratio.
The efficiancy of an RO membrane is dependant on water temp, pressure and the amount of TDS present in the source water. After it passes thru the RO stage, it has most , but not all impurities removed, thats where the DeIonizing stage come in, water flows thru this stage and passes thru resin beads that come in 2 types, anion and cation, attracting positive and negative ions to the beads, allowing near pure water to pass thru.
Now that you have stripped all the nasties out, you have to turn around and replace the calcium, mg, and other stuff, but you want to control those anyway What you have done is removed a bunch of nasties, that while acceptable for muni drinking water, can lead to long term problems with delicate inverts. Municipal water supplies add a lot of stuff thats harmful to a reef tank long term, and can be a large source of phosphurous compounds
Being on a well is prolly not much better, if you are in a rural area, you have septic tanks, fertilizers and pesticides,herbicides, petroleum from road maint and dumping after oil changes. These are all potential contaminates leaching into your water table
At $.35 a day for store water, within a year you are putting out enough to get your own, where you control the quality and amount . What if you need to mix up a trash can full for an emergency water change, or get a new tank, start off with the purest water you can and you are so far ahead of the game.
Even at $200, thats a fraction of what you will invest in the typical 55-120 gal reef, but that is prolly the leading source of long term problems encountered in reefkeeping