I can’t tell if they’re weak looking or not. It’s literally just 3 torches. Maybe my nutrients have just been too low but everything else is fine including 1 torch other LPS and sps so far. So very odd I would think to just have a few peices regression
I was referring to the reason the thread was opened, which is the corals you mentioned as being retracted. (Figuring out the phosphate readings is coincidental, right?)
Coral nutrition is an interesting subject becuase it all revolves around something called the "coral diffusion boundary layer".
Corals don't experience the water around them directly, they experience it though this boundary layer. Flow characteristics and the corals genetics (species) determine this boundary layer.
Because corals are different from one another, and because different areas in the tank experience differences in flow, individual corals have different experiences of the same water.
Corals in lower flow areas will have a lower rate of access to the water, in general.
Similarly, some corals are built to have a stronger boundary layer to withstand more flow, others are built to have less boundary layer because they don't experience much flow. Our tank can only be one kind of way, so all corals are not equally satisfied even if all of them are "fine". And in fact "fine" is a range, and some will be at the top and the bottom of that range....hence (IMO) your retracted corals.
Anywho....keeping PO4 and NO3 levels above a certain level is to avoid having to worry about these ranges too much, so that it's easier to make *all the corals* happy, even if our flow isn't perfect for *every* coral. But keeping great flow is also still a top priority for this AND other reasons.
A corals' feeding status matters here, but feeding corals without wrecking the tank is usually pretty hard....so emphasizing dissolved nutrients (and cleanup crews!) is usually the way.