Bryopsis died once the PO4 fell below ~0.15ppm, in theory a skimmer wouldn't change much for the issue I had.
Good to know that about the Bryopsis. That's a great data point. Not sure I agree about your skimmer conclusion though. But don't have enough knowledge to speak comfortably on the matter.
However...
...I bet with all the liverock people use their traditional tanks, they too become nitrate limited!
Being Nitrate limted was caused by an excess of Phosphates. And live rock does not leach Phosphates. Dead rock can though, at least until it is cured. And in a tank... that could be months. Your current PO4 reading, is that in a tank with the rock I gave you?
If so, it's certainly possible that I got my fully cured and partially cured dead rock mixed up. If so, that could be a source of your Nitrates I'm afraid. Sorry.
But peoples efforts to do water changes to keep N's & P's down is futile! Its crazy to me people even do water changes!
N & P reduction is only one small reason for doing water changes. And IMO, anyone who's doing them with that as the principle reason does not understand the hobby. Regular W/C's - unless done on a silly large scale - are a poor way to achieve significant N & P reduction.
W/C's replenish trace elements, and keep many, many things that would be destructive to corals at an acceptable level. Contrary to what some appear to believe, algae will not absorb all the bad stuff that makes its way into our tanks. As an example - sliver. Granted, that should never been present in thing beyond low trace levels. But it's bad for corals and really bad for other photosynthetic organisms too. Like algae. And algae is not going to take in a poison. It's not a sponge, taking up anything in the water. It's an organism that only takes in things that are good for it. IMO that's why we need W/C's. Not N & P reduction. And while a skimmer may only pull out DOC's many of the compounds in DOCs may well be things that algae does not want either.
BUT it would be fascinating if I were wrong! Can you point to any reliable proof of a reefer that grows SPS (the more sensitive variety of coral), and have done so for years successfully, with no water changes, and no filtration other than an ATS? If you can point me to such evidence, I've got a ten dollar bill that I'll hand you at our next meeting.
Thanks for the reply! Interesting subject. And congratulations again on figuring out the problem and fixing it!