Java fern leaf tips transparent

[Brown_D at pcfnotes1_wustl.edu]

I checked a few online lists of plant nutrient deficiency symptoms, but
none sound exactly like what is happening to some java fern leaves:  the
overall plant is lush and growing rapidly, but the tips of a few older
leaves are becoming transparent without not turning color.  No other plants
in the tank are doing anything similar.  Lighting is 2.7 WPG, fertilization
with PMDD dosed daily, injected CO2, water changes 40% weekly.  Leaf
changes noticed around the same time that 1-3mm white conical shelled
snails appeared in the tank (botia striata notwithstanding).  Does this
sound like a nutrient deficiency?  Snail damage?   I have no web page to
post pics but can e-mail directly if anyone would like to see them.

thanks in advance for your help

Diane Brown in St. Louis where we're going to have a freezing, but not
white, thanksgiving