Craig,
I struggled with my mixed saltwater too after getting my RO/DI unit up and running.
I tried a lot of different things - but was unhappy with the results.
I use Instant Ocean brand saltmix.
Finally I called Aquarium Systems - the company that makes Instant Ocean - and talked to one of their tech guys.
I'll try to repeat what he and I discussed, what to do and why, when using RO or RO/DI water for the freshwater source.
Water that leaves a municipal water treatment plant often times has a lot of CO2 gas in it. It travels to our homes in pipes. The CO2 gas has nowhere to escape, and stays in the tapwater.
When you run this tap water through an RO membrane, most of the bad stuff is removed. Some of the good stuff is removed too, such as bicarbonates and other minerals. These carbonates, bicarbonates and minerals tend to keep the pH of the freshwater up above 7.0
My tapwater measures out around 8.5 to 9.0 - very high for tap water. Lots of minerals and stuff in it I guess.
When you remove these elements and minerals - you're RO water should be mostly free of them, and the pH will measure somewhere around a neutral 7.0 or so.
Since you removed most of the bicarbonates, carbonates and minerals - you're alkalinty should be very low as well.
You've made a good batch of RO water, with neutral pH and very low alkalinity ..... which is really great for mixing with the dry saltmix.
Here lies a very big potential problem though;
That CO2 gas that may be in the city water / tap water - does not get removed. It can pass right on through the RO membrane. All of it does not pass though, but a lot can.
So this new batch of RO water you think is ready to go - isn't ready yet. It may still have a very high concentration of CO2 gas still in solution.
CO2 gas in saltwater will decrease the pH.
CO2 gas in a freshly mixed batch of saltwater will decrease the pH as well - and cause a loss of alkalinity.
If unaware of this occurance - when you mix new saltwater, you find that your measurements don't look so good - and it makes you scratch your head wondering why.
So all you have to do is aerate the RO water overnight before mixing the saltmix into it.
Run your RO unit as you normally would into your mixing container, tub or bucket.
Toss is a powerhead. I lay mine on the bottom of the tub with the jet pointing up. Plug the sucker in and let it run overnight.
This will allow the "excess" CO2 gas in the RO water to escape from the water, and it will finally reach a point called equalibrium.
There's still some CO2 gas in the RO water - but it's where it should be, or more accurately said - under the surrounding air's CO2 gas concentration, it's where it's allowed to be.
Next day - slowly pour the saltmix into the water, stirring it with a large plastic spoon. Add enough to get your salinity the same as your display tank. I like to mix mine to a S.G. or 1.024 at room temperature of about 75F.
Now let it run overnight again with the powerhead going, and a heater if necessary. This practice is very common in the hobby, and insures that the salt and minerals are dissolved well, and allows the chemistry to become stable. Again it's mostly a gas issue here - making sure the saltwater again reaches it's new equalibrium.
Next day - do your alkalinity, pH and calcium tests on this batch.
I bet you'll find your pH is at 8.2 to 8.3
Your alkalinity is around 8 - 9 dkh
And your calcium levels are near 380 ppm.
Or very near these levels.
No need to tweek the values of the new saltwater in the mixing tub. If you have the above readings, then it's about perfect as far as the tech guy told me.
If you do start messing around with it, adding additives or trying to tweek the levels, most likely you'll do just that ............. mess it all up.
It's normally better to adjust any alkalinity, calcium or pH levels in the display tank - slowly.
I wish you much luck ( but luck has very little to do with it )
