Inefficient is an understatement and sets a value to ro/di water. As a result tho, I have reconsidered this 70%, or 50% and even a 30% monthly water change. I have a 90 reef tank, a 50g refugium, and a ~25g sump. The sump has a protein skimmer and that is it for filtering. I take out 0 to 9g a day at 3 gallons per siphon. This results in a 5% to 20% water change per week. I add salt directly to the center chamber of my sump proportional to the amount of water removed at that moment. The spec. grav. varies very little & the chemistry is spot on.
It does bother me that the salt does not sit and fully dissolve as many have indicated as a minimum into something like a 30g mixing barrel. But, I have been doing this for several months with zero issues.
I have quite a variety of plants in the refugium. So much so that the plants compete. I hate loosing a variety, but it does give perspective about the system chemistry. Certain varieties are definitely far more efficient at removing nitrates/nitrites than others and they flourish. Others die out completely and others "hover". I do not like losing a plant any more than a fish, but this stuff simply is just documented.
I also have a bazillion pods in the ref ... not to be too scientific in the count. But there is never any algae on the ref tank walls. There are a bazillion copepods tho and they do eat algae. There are also many amphipods which are funny and weird little critters. To all of that I added a mandarine goby (they only eat pods) which is considered "difficult". I rarely see the guy, but it is definitely not starving. Maybe a bit lonely tho.
Let sum up that I love my ro/di auto-top off water supply. I never give a thought to getting water. It is as good as you can get and is only a part of the insurance policy toward protecting the investment.
