John, if you wait to get a bad reading it may be too late. I say to do a water change on a regular schedule and take no chances of water quality going to pot. I say you will be better off to keep the water changes going instead of waiting for something to go wrong and then your in a hussle to get RO together and get it mixed and get it changed. There are so many differing opinions on this. M lacom99, you just need to test it out and see what works best for you. I know alot of people who do a 10 gallon monthly water change on a 100 gallon tank and have great success. I also know people who have tried to get away with no water changes and there tank has crashed. Your friend isn't doing a water change because when water evaporates it leaves everything solid behind. As birdlady told you the salinity levels will rise as water evaporates because the salt is not leaving the water it's staying behind. So is ammonia, nitrate, nitrite and phosphate. Keep us posted on your decisions.