The "RO/DI strips the good stuff out" argument is bunk. Any reef salt mix puts everything you need into the water. What a salt mix can't do is remove harmful stuff (excepting chloramine) from your tap water.
I'll explain a primary reason for using pure water:
- note, this only accounts for one element (phosphate) others such as silicate have similar issues.
Aquariums are closed systems. Without human intervention, nothing enters and nothing leaves. Elements can't be broken down any further than the elemental level (excepting fission reactions) They can, however, bond chemically with other elements into different forms which are more or less available to living organisms depending on their chemical makeup.
Why is this important? Well, studies have established that the primary limiting factor in reef systems is phosphate. Corals grow in biotopes in which the majority of phosphate is bound in existing organisms. Consequently, corals don't need a significant amount of bio-available phosphate, nor can they use it. In fact, the presence of phosphate in high levels inhibits coral growth (hence agricultural runoff destroying natural reefs). However, what we consider
nuisance algae is exceptionally well suited to using free bio-available phosphate in the water column.
Phosphate is present in all organic matter in some form or another. Algae consumes free phosphate in the water column and binds it into a form which is unmeasurable by chemical water column tests. When dead algae (and other organic matter such as fish poop) decompose, the phosphate is released. However, it is quickly consumed by living algae, leading to cycles of nuisance algae.
In an ocean reef, phosphate is removed from the system through dilution. In a closed system such as a reef tank, there is no way for phosphate to leave the system excepting human intervention. We use skimmers and water changes to remove dissolved organics before they break down into their component compounds (notably, nitrate and phosphate). This is one way of managing nutrient levels (i.e. export of nutrients). However, export can only remove a small degree of organics. Therefore, we must consider the amount of nutrient we are importing into the system. All organic material we put into the system contains phosphate and
nitrogenous compounds. Feeding is a large source of nutrient import, hence the recommendation to underfeed our tanks.
The other large source of nutrient import is tap water. Most well and city water contains a significant amount of dissolved solids (phosphate and silicate being most significant for our discussion). On avarage, american public potable water contains 200-300 parts per million (ppm) of total dissolved solids (tds). Certainly some of it is "good" for a reef (notably, magnesium, calcium, and strontium), and that is indeed stripped out by ro/di. However, as mentioned before, those elements are replaced by any reef salt mix.
Because of the potential introduction of high levels of phosphate and silicate, which feed respectively algae and diatom blooms, and further because of the potential to introduce harmful chemical compounds found in tap water (chlorine, chloramine, ammonia, and some heavy metals), it behooves us to add only pure water into our tanks and add trace elements as needed (primarily via salt mix, subsequently through testing and addition as needed)
To sum up, there are three main points:
1) Tap water contains chemical compounds which promote nuisance organic growth, and inhibit coral (particularly calcareous coral) growth.
2) RO/DI water is neutral, it is essentially free of all extraneous elements and chemical compounds regardless of their benefit or detriment in a reef environment.
3) The popular salt mixes on the market today add all necessary trace elements and condition water to 8.2-8.3 ph.
We are left with one question, why would you ever add things to a closed system that you will have to remove later?
Think of it this way: If I swallow some water with a toxic compound in it, I can have it removed through vomiting, sweating, and excreting (not to mention stomach pumping) but why should I swallow the toxic water in the first place if I can cheaply and easily purify it?
That is the short, non technical explanation...some of our more science minded individuals (spanky and tdwyatt in particular) have given exhaustive explanations which go way beyond my level of knowledge, and are available here. In particular, there was a think tank discussion of the week on phosphate which was very informative.
--- Not bad for my 500th post, eh?
