Gokul: "i am sure this is something to do with substrate disturbance"
as do I, I get it sometimes if I vac to hard in one spot. trying to pick up some stray moss
jfynyson: "I believe this kicked up some ammonia" "however this time I had the substrate disturbed as well
TekWarren: "others report it will bounce back if the root systems have established" "These cases do NOT seem be the result of "converting".
1) mine always does. 2) my tank is over 2 years old, so NOT converting!
tenthe: "The idea of a chemical message makes sense intuitively, it would explain why the browning occurs in patches."
that kind of makes sense too. after 4 months planted, at the start of last summer this happened. nothing all winter, now I'm just getting over some spots here and there
sorry for all the quotes people
in my case I it's very hard for me to vac under all my s.repens (I have a lot as you will see) my first thoughts was "Hydrogen Sulfide "
being a very low to the substrate, and my 3 1/2 inch bed.
bacterial breakdown of organic matter and a lack of of oxygen!
every now and then i'll see a large bubble pop up from under the plants.(?)
(this is why I think this way)
this could explain the die off for me, but not the OP. (first 4 months, little vacuuming. for most of this winter, even less!)
but I must say never as bad as his
so now every WC and vac I aerate the "lawn" with a chop stick.
open the air intake every night on my powerhead.
and the bad spots are less and less as time go's by.
but still happen now and then. be it only to the large underlying leaves.
photos of past decay:
left alone all spots coming back
sad thing is, all this. and I really have no advice for the OP. other then to say let it be and see if it comes back.
mine always dose
o2 powerhead with home made skimmer