Instant Ocean is the commercial salt mix closest to real ocean water. However, an aquarium puts higher demands proportionally on its water than a reef puts on the oceans waters. Vast ocean versus little bitty tank, therefore, most other salt manufacturers use more calcium compounds (Buffers) to make up for the aquariums greater alkalinity needs/problems. Instant Ocean is steadfast in its sticking to close to an ocean mix, that and it sells huge amounts of its mix to universities and private research houses due to its near to ocean mixes. Generally aquarists just mix a different salt mix with Instant Ocean. It is usually advisable that you add buffers to RO water when using Instant Ocean mix whereas most the other major makers of salt mixes usually add extra buffers to their mixes already. Instant Ocean also makes Reef Crystals salt mix which addresses the buffer problems with its regular mix when used in Reef tanks. Unless you have a heavy live rock, coralline algae and hard coral tank the Instant Ocean issue should not be a problem for you. Personally I have been using mostly Instant Ocean since 1972. I have tried others but consistently return to Instant Ocean. I have less sediment problems with it and less problems with nitrogen and phosphorus with Instant Ocean than with the other mixes. I use Seachem Marine Buffer in very small amounts with my RO water for mixing up makeup water, even when using the water to make Kalkwasser. I adjust my alkalinity (buffering capacity) of all my make up water, not the pH. Seachem Reef Buffer adjusts your pH without generally having much impact on your alkalinity (nor does it increase alkalinity much). As far as changing water, check your phosphates and preferably use a phosphate removing pad or bag of Phosban. The only other thing you might have to worry about is a high silica content. Before you use any more tap water test it or take it to a water softener sales house and they will generally test it once for free. Also distributors of domestic water have to make public the results of the tests on their water, before and after its treated.
