Questions on GH

[Jman17H]

I am looking for an overview and opinions on the importance of hardness (GH) when it comes to planted tanks.

What is the perfect level I should strive for and how do I keep it there once it is achieved?

My tap has varied from a GH of 6-8.

I also have this:
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=4484&N=2004+113775
but after using it last night, I think maybe my cartridge is bad or something for the layers were separating and a few green pieces were going into the bucket.

I have 3 tanks planted right now with the following GH:
20 tall - 8
30 long - 18 before last nights 50% water change with previously mentioned RO
150 breeder - 25 (I know way to high and not sure why?) It has 2 large peices of driftwood and a bunch of granite w/ one questionable metallic rock that I am taking out tomorrow.

I have sort of been lazy with my caring for GH for several months now.

What level should I be shooting for and why and how?

 

[Zezmo]

That filter will not affect your GH. In fact, a filter like that probable does a lot less than they suggest it will. I imagine the filter is worn out in less than 100 gallons (my own estimate). You need an R/O filter to remove GH components.
GH is typically calcium and magnesium for the most part. Both critical plant nutrients. Anything above 2deg GH should be fine. I run one tank with a GH of 15-20 (my shrimp tank) and plants thrive in there.

Don't shoot for any level. Your tap GH sounds fine, dont mess with it.

 

[Purrbox]

Your tap level sounds perfect. As long as you dont' start to see signs of either calcium or magnesium deficiency I would leave it alone.