Agreed with the above. A significant conceptual issue with so-called ULN tanks is that almost everyone (if not everyone) who is purposefully running an "ultra low nutrient" tank has absolutely no idea what high vs. low inorganic nutrient supply is like relative to corals in nature. The very lowest levels of detection with hobbyist grade kits are in fact rather high nutrient levels compared to a natural reef. Hence, undetectable nutrients could mean that nutrients are on the high side, but undetectable, or they could be extremely low. Since they are undetectable there is, by definition, no way to know.
A major factor which is entirely or almost entirely unappreciated amongst reefkeepers which controls nutrient supply to corals, algae, etc. is water flow, and hence mass transfer. The corals don't care one iota how much nitrogen, phosphorus, etc. there is in the water, they care how much they are receiving. At low flow rates, high nutrient concentration is required to provide high delivery of nutrients to the corals. At high flow rates a much lower concentration of nutrients is required to deliver the same amount of nutrients (as related to changes in diffusive boundary layers and mass transfer).
I'm reminded of a conversation I had a few months ago with Marlin Atkinson, one of real experts on coral reef biogeochemistry. Marlin was running some experiments in a flume (i.e., experimental aquarium) which by reef tank standards had pretty strong flow. However, the corals took up nutrients much more slowly than they do out on the barrier reef flat here in Kaneohe Bay. In order to get the corals to take up nutrients at about the same rate as they do in nature he had to increaes the nutrient concentrations by nearly an order of magnitude which compensated for our inability to provide the kind of flow that corals in these kinds of high energy environments receive.
Suffice to say, it becomes a complicated endeaver to compare reef tanks to natural reefs. Measured parameters are a very useful guide, but they need to be coupled with careful observation of the critters. Whether the nutrient supply "feels" high, low, or in between to the coral depends on multiple factors and the nutrient concentration alone isn't remotely sufficient to describe what's going on.
cj
