Hey there,

I've had exactly the same problem in my highlight (4 w/gallon T8 11 hours/day) 30 gallon tank about a month ago. All my plants were thriving, except HM and Mayaca. A small percentage of stems will either grow stunted or rot away. I eventually solved it by doubling the recommended nitrate and phosphate dosage, from 1/4tsp KNO3 and 1/8tsp KH2PO4, to 1/2tsp KNO3 and 1/4tsp KH2PO4. However, my fish didnt respond well to the change, so I've decided to drop the dosage back to the recommended levels, and reducing my photo period to about 9 hours/day.

So I would agree with Tom and try increasing your KNO3 levels.

Hope this helps.

Regards,
Steve