This thread caught my eye. An old, retired home based coral farmer, I started selling coral for profit in 1996. Over the years, I occasionally re-evaluated how to ensure the fastest growth and profit for the least amount of maintenance and lowest cost. If that describes anything like what you are thinking, I will attempt to describe my routine below. If this is more than you were looking for, at least it may give you some ideas and/or a different perspective.
Testing
Alk and Ca - approximately weekly
Mg - approximately monthly
PO4 - about every two months until I figured out how to judge PO4 level by viewing the color saturation in the Activated Alumina(AA) granules used to remove PO4 (AA is easier and cheaper than GFO)
Dosing
- Alk, Ca and Mg were dosed as test results indicated. In my last and best coral farm(300 gal total), about half the coral was stony coral and it grew very fast. There was also a lot of Coralline Algae growth. I was going through almost 2 liters of homemade Alk (baking soda and baked baking soda) and about 1 liter of Calcium (CaCl2 ice melt) weekly. The system used almost twice as much Alk as Calcium. I believe this was due to the use of a special substrate which provided high Ca as it dissolved.
- Trace elements were dosed with tablespoonfuls of Kent Coral-Vite alternating with Kent Essential Elements almost weekly. Also added Iodine from time to time.
Since I was working in or at least checking on my farm every day, I chose to stick to mostly manual dosing about twice weekly though at one point I set up automatic dosing of Alk. I found early on that in a heavily populated farm, water changes could never keep up with demand for Alk and Ca. I also found that by dosing the main three components along with the most consumed trace elements, water changes were practically unnecessary. (There are excellent experienced hobbyists in this group that do water changes once or twice yearly, and, they never speak up here on the forum, possibly for fear of reprisal by over zealous, less experienced hobbyists.)
I came to understand that the main component in salt water is sodium chloride(salt), with Alkalinity(though actually not an element), Calcium and Magnesium following in that order. Since salt never evaporates and is never used up, it seemed logical to leave salt alone and instead focus on making sure the components and elements down the line from salt were kept at proper levels. This could only be accomplished by testing and dosing.
I found that after all costs were tabulated, using just salt water was way too expensive. The combined cost of Test Kits, Baking Soda(Alk), Ice Melt(Ca), Epsom Salt(Mg), Kalkwasser(if used), and Kent trace element products was much, much less than if I tried to feed those things to the system via new salt water. In fact, the only way that exchanging salt water could keep up with demand was to do approximately 10% daily water changes. Whether continuous feed or batch exchanged, this was just too much effort and too prohibitive cost-wise.
Something I never did use in my coral farming was a Calcium Reactor, though it would have been my next step on the 300 gal farm which I had to shut down prematurely after 3 years due to family concerns. I had set up Calcium Reactors for many clients. I probably never bothered because I liked keeping my finger on the pulse of my coral farm rather than using automation which can be subject to malfunction with horrendous consequences. Coral is quite forgiving when Alkalinity gradually falls to minimal levels but goes into a death roll when it is dumped in.
There it is, for what it's worth. Hope it helps.
Aloha,
Mark