No SPS growth... stumped

Sure, sand beds can also leach metals and such. I like sand beds though, and personally have run them for a long time without issue. I think as long as you take care of the tank it isn't much of an issue. I've actually found BB tanks to be much more of a PITA, but for different reasons.

The issue with sand is two-fold though.
1st, stuff can settle into it. So, best to have good flow and filtration and keep crud suspended so it can be filtered out. Also, good to siphon the sand lightly IMO.

2nd, the sand is constantly dissolving and re-crystallizing. So, phosphate and certain metals can bind to it. Then, they can be released latter. However, this is mostly an equilibrium process, so, if levels are kept low, it shouldn't be a significant issue latter. The problem is when you slack on maintenance, then it can take longer to remove, as your sand has become a sink. If that's the case, running GFO will certainly be sufficient, if levels are kept low. Though, it can take a while and a fair amount of GFO.

 

Right, your second point has been in the back of my mind for the last 8 months. Would the fact that this sandbed was in a tank where PO4 was constantly in the 5-10ppm range for over 2 months mean it absorbed a large amount of PO4, to the point of it now releasing back into the water column?

I dunno, but will test and observe for a little while hear. Good discussion though, definitely learning things here! ;D

 

yes, certainly that is likely.

 

Rust?

Interesting find tonight. Was cleaning out my 35 gal rubbermaid saltwater mixing/holding trash can, and there was a deposit on the inside bottom 12 inches, almost like the calc precipitated out of the saltwater. When I rubbed it, it wouldnt come off, but a thin layer of dark brown "powder" rubbed off, looking exactly like rust. I looked, but did not find anything that might have dropped in and started rusting.

I clean the can after every bucket of salt, so about 170 gallons ago, or about 3-4 months ago, and I remember the calc precitipation was there, but not the rust powder substance.

How bad of a reaction would corals have to traces of rust?

 

I doubt it is rust. A lot of salt mixes precipitate out "vitamins." Basically this is just trace minerals being left behind. Reef crystals is notorious for this, and a quick search on google will show you tons of people who have the same stuff left behind. Usually completely harmless.

 

Very interesting. Could have sworn it was rust, had that gritty texture... but alas, perhaps not.