I have just started using chemi-pure and have been extremely pleased with it but dang its expensive. If I do add strictly carbon, I'll usually add the black diamond carbon simply because of price point. I was wondering what others thought about the black diamond.
All carbon leaches phosphate, but some brands leach far more phospate than others.
I used to use Seachem's Matrix carbon - but only because it was the only top grade carbon available locally. I'm sure all of the top grade carbons will be much the same.
Carbon removes some trace elements very efficently - Iodide springs to mind.
The synthetic equivalents like Purigen and Chemi-Pure act a lot like carbon but have a minimal impact on trace elements.