Warning- bad carbon will kill

[fencer]

There are different pHs for various prepared carbons. I don't think it is the iron as iron salts are an additive in the Ultra products. I suggest it may be something else in the carbon another contaminant? Was there a large spike in algae growth, did any of your yellow sps start turning greenish/yellow? These are signs of overdosing of iron

[rayjay]

A lot of hobbyists dose iron for their tanks to help growth of macro that they have in their tanks.
One japanese hobbyists uses a jar of nails in his sump from what I read on a post on RC.

[kien]

ya I was kinda wondering that too because GFO is more or less just that, rust? Things that make you go hmmm....