Water Sprite

I grow lots of water sprite because it grows fast, looks good and is a 
good plants to jump start plant tanks.  I have two tanks that are 30" 
tall.  Got these after noticing that many plants keep right on growing 
and would have gone taller but I'm vertically impaired with a short reach 
to boot and anything more than 30" means the snorkel gear comes out.  

At any rate, the water sprite will in time grow right out the top and 
when I pull these monster plants to throw them away the plant is 
typically 24-30 tall with 8 inches of root.  

Here's my question.  After it is growing for several weeks some of the 
stems develop an area that looks like something is eating them.  Can be 
anywhere from a quarter inch to several inches long.  In extreme cases 
the stem looks like is is being eaten through and will separate.  
Doesn't happen to all w.s. and may be on new 
fronds or older fronds.  Not bad enough to stop the stuff growing but 
indicates a problem of some kind.  

Tank has MH lites, PMDD, co2 etc.  pH runs 7.2-7.8.

Anybody know what  causes the stems to look like they are being eaten?

--Earle Hamilton from northern Michigan where coral once grew