>The tank has a persistant low level algae problem and I've tried everything.<
That and cyano are the warning signs. You saw what was going on in the tank, yuck, and I hate algae. And I hate fighting it. For us, it's just a matter of getting off our lazy rear ends and getting new rocks and sand.
>How do you know that the live rock has become a nutrient sink? Is there any way to tell? How does one check for this?<
Any calcium carbonate material, just by the very nature of it, will act as a filter. Keeping your nutrient imports at a level that bacteria and critters in the rocks and sand can keep it moving for you will work. The reality of it is that it's really hard to do. It's ability to ad/absord and also such a high ability to store so much of it before it becomes full, covers a multitude of sins. By the time you start seeing problems (algae, cyano, etc), it's full.
>I know you were de-gunking some of the live rock in a dark container with a powerhead. It looked like a lot of detritus fell out of the rock and into the container. Would that rock still be good or do you think it was still plugged up and needs to be replaced.<
Storing it in the dark, water changes, that's a old way of cleaning up rock and it works. It gives bacteria time to drive these things out of the rock. I don't think it will make it brand new again, but it will definitely clean it up a lot. That's why it was advised for so long to store "new" rocks in the dark and do just this. Now that rocks with "all this life" have become popular, people have steered away from that.
>Maybe this is the next step.<
Try putting the rock in the dark for a few months. Siphon out the detritus do big water changes on it. When you see very little detritus being produced, you're good to go again.
Bob, you know how this tank looks in the house. When it's not clean, the whole house looks dirty to me. With just fish, I can keep it sparkling!
