Activated carbon is great for filtering water, which is why it's used in many potable water filtering systems. It also has practical application in aquariums and has been used for over 50 years that I know of. Activated carbon adsorbs all sorts of 'impurities' that would otherwise lower the quality of the water. However, carbon is a little costly, has a short use-life, and is not renewable. In addition, it is somewhat counter-productive in a planted tank as some of the 'impurities' it removes would otherwise be nutrients for the plants. Also, as mentioned, sufficient partial water changes are far more effective at pollution reduction than any activated carbon regimen. The simple truth is that activated carbon may have some benefit in certain fish only tanks, but less benefit in planted tanks that receive routine partial water changes.
As to the use of Purigen...I used Purigen for quite some time and frankly feel that it's less effective at water polishing then simple polyester fiber (I buy the big bags at WM). In addition, I think the marketing hype that Purigen removes organics that would otherwise create nitrates is misleading as it's likely that nitrates are created earlier in the decomposition process and what's being trapped is the more inert microscopic sludge.
Also, I have long since questioned the accepted hobby hype that we need 4x to 10x GPH water flow through filters for sufficient filtration. I even saw some 'advice' that suggested even more for planted tanks. Additionally, I've read across forums of many that 'brag' about their 'over-filtration' with extra filters, powerheads and such.
It seems to me that logic dictates that good water filtration is about how well we filter the water, not how much water is forced through a filter. That said, a finer filtration media at a slower flow should produce a cleaner result. Of course we want good water circulation in a tank, but it doesn't need to be a raging torrent.
So I guess what I'm saying is that if you filtered every drop very well a couple of times an hour, it should be good!
Having 'said' that, I'll confess that from time to time, I'll install my Marineland Internal Magnum Polishing Filter (I'm not affiliated with Marineland in any way) with the micron filter cartridge (with or without diatomaceous earth) to remove even the finest particulates. I'll let it run for 4 hours or so.
Finally, since I'm on a bit of a roll, lets talk about bio-media. I used to be a 'fan' of Matrix/DeNitrate (a glorified lava rock pumice stone). After years of testing I've resigned that it is no more effective than simple bio-sponge. It's "claim to fame" is micro pores that claim it creates more surface area. But these micro pores plug all too easily with detritus, making it far less effective. It also claims that these micro pores will host anaerobic bacteria that will reduce nitrates. I never found this to be the case, even seeding with fair amounts of Stability.
Just my results - yours may vary....but I've migrated to simple sponge material in my filters. Incidentally, there's far more beneficial biology in the substrate and on hard surfaces in the tank than would ever be found in most filters.
My apologies to p0tluck if this took us [way] too far off the clear water path!