Chemipure is ok but it definitely does not keep your water clear for their advertised 6 months. Chemipure becomes more of a biological media rather than a chemical filtration. For polishing your water I find a sack of carbon changed every few weeks to be much more effective.
All the "suggested" durations on bottles of anything for your tank are highly relative. A 75 gallon that's overstocked with eels and triggers is going to go through it much faster than the same tank as a reef with a light fish load. The same goes for trace elements and other additives and filtration. Phosban lasts for a long time according to the label. Well, if there is no phosphate in your tank it may last that long, but on the same note if you have high levels of PO4 you may need to switch it a lot more frequently. Also, I'm pretty sure chemi-pure is carbon last I checked, I may be wrong.
I personally started using the PURA complete bags in my filters and I love the stuff. Magnavore makes it.
yeah its like the nitra zorb i use in my cichlid tank, it says it lasts about a month and a half, but mine is gone in 2-3 weeks if i change it every 3 weeks, less if its every month, and my cousins lasts him about 2 months keeping his cichlid tanks nitrate down at 10ppm with a lighter fish load.
I love chemi-pure.
For nano tanks, its almost a must have.
In my case, a 1/2 sack lasted 3 months (7gallon nano).
It kept my water uber-clear and I believe it contributed in stabalizing my pH.