The only time it is prudent IMO to run carbon is either after you have dosed your tank with medications or when you are acclimating a new substrate like ADA AS, but once established and dosing ferts, I do not see the need to run carbon, ever.
Never use carbon on my planted tanks. I've seen more than a couple comments that people use Purigen though. I've also seen posts from Seachem reps saying Purigen has a minimal impact on the nutrients we add, so I'm planning on tossing a bag of it into my 75G when I get it setup. Atleast for the first month or so, if not longer. Seems like the first month or two is always the toughest (algae blooms/spikes).
I just read up on the Purigen- Looks like it's very good at removing Ammonia, Nitrires and Nitrates. That doesn't sound like it would be great for the plants. Does anyone have problem with superlow nitrates?
I had the same questions about it and Seachem's response was:
It does not remove ammonia/nitrite/nitrate. It removes the nitrogenous waste before it gets broken down into ammonia and thus nitrified. If you are adding nitrates via dry or liquid fert, you will not be affected at all. And if you are not using additional macro ferts, then it is still possible that you will have enough nitrate from fish waste to sustain a small-medium plant density because it only removes a percentage of the waste unless you use way more than is recommended.
i'm jussing purigen now in my tank (dutch tank, lots of plants) for about 6 mouths and call tell that it will not remove nitrate or thinks like that.
will only make you water more clearly witch is good for plants that are grwoing close to the ground. more light will get to them.