Here's a detailed explanation.
When you make sea salt it "usually" takes 3 cups of sea salt to 5 gallons of water for reefs, but for fish only, it only takes 2 1/2 cups of sea salt to 5 gallons of water, so when they say "watered down salt water" it means that they didn't put enough salt mix per gallon of water, got it? Oh and the water has to be clean, like RO, or deionized water! ( this is only an example, for different salt mixes use different measurements.)
I recommend that you get a refractometer! and make sure your reef salinity is at 1.025, but no greater 1.026, and some kind of ATO (auto top off) meaning that as your water evaporates, water is being replaced so that your salinity doesn't fluctuate. note that when water evaporates, the salt doesn't evaporate so when you replenish water, make sure it has no salt at all. The only time you add salt water to your tank, is when you do water changes. Hope this is helpful.
