health Leaf tips yellowing with black spots

[stickyfingers111]

I'm noticing one of my plants has a lot of leaves that are showing yellowing around the edges with black spots. I think it's a nute issue, but I'm not sure which nute. Can't seem to match this up with any of the charts that show typical nute problems and what they look like. Any ideas? My setup is as follows;
 
- DWC in 5 gallon buckets
- The plant is an apocalypse red adult and has plenty of pods ~30
- Running CNS 17 grow at 650 ppm
- Nute solution stays around 68 degrees F using icepacks
- I use a recirculating fan for airflow
- Airstones and pumps are working fine
- I dump and mix up new solution every 2 weeks (top off water and add nutes when needed between changes)
- I keep the PH at 6.3-6.5
 
Here's a pic of one of the leaves;
 
 
 

 

[Spicytigger]

http://thehotpepper.com/topic/73110-help-identifying-issue/