explosive growth, pH crash, what happened?

Hi,

i've had a planted tank for about 3 years now. over the past year, most of
my plants died off (ozelot, crypts, etc), and the two remaining amazon
swords were looking pretty wan, and i had expced it to completely die off
leaving the tank just with anubius and java fern. since only the low-lights
are doing well, I hadn't renewed the lights either, and only fertilized once
in a blue moon (slow growth plants only)

however, in the past 2 months, the almost-dying amazon started to revive
itself, and basically went from a 4" puny, sad-looking plants to a 15"
bushy, healthy plants. to support its unexplained growth, I have increased
the fertilizer dosing. now the java ferns have leaves about 7inches long
and 1.5in wide, and is growing like a weed too.

I still don't understand what happened that caused the sword plants to
experience such amazing revival. I have not done much in the tank (37G
tall, 55W, fish, UGF, HOT Magnum) other than periodic water changes, and
only increased fertilizing/excel after it became apparent that the swords
were going through some sort of a revival. of course, this renewed growth
caused a pH crash to 6.0 since my water change cycle was no longer frequent
enough (my tank is very soft.. usually is like kH=2 or so.. now it's near
zero. I guess the plants ate up all the minerals), so I'm in the middle of
trying to stabilize it, but that's another story.

does anyone have any clue what may have changed in the tank to cause this
incredible change?

linda