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Tapwater Phosphate, again

Hello, all,
	I've been lurking for a couple of months and perusing the archives, but
thought I'd put this out to seek some specific advice.  I had a 135 gallon
planted tank running for about six weeks.  I'm injecting CO2 (18-20 ppm
using a RedSea CO2 kit) and have about 220 watts of fluorescent light (2 40W
Chroma 75, 2 40W Chroma 50, 2 30W Hagen Life-Glo) running off electronic
ballasts.  The substrate is #2 gravel with 3 packages of Duplarit-G in the
bottom third (no cloudiness problem for me, though) and bits of Fern and
Palm (low P) plant spikes under the plants.  I've been keeping it at 82 F,
and the pH stays around 6.5 (I cut back the CO2 injection at night by half
with a multi-event timer.)

The tank is fairly heavily planted (thanks, DanQ) and the plants are doing
well, esp. the Hygro. polysperma, and Myriophyllum, both of which I've
already had to prune severely.  The Aponogeton ulvaceus and red-edged
radicans sword have flowered, as well.  I've been trying to toe the PMDD
line, but can't find a local source for the trace elements, so I've been
using Kent Freshwater Plant plus a solution of KNO3 and Epsom salts.  (K2SO4
and CSM are in the works...)  Fish load is light, with just three
Apistogramma and a couple of plecos.

	Like so many other New-Tankers, I've been getting some filamentous algae
growth.  At first, I thought this was due to P release from decomposing
plant and fish detritus so I started doing small water changes (~5 gal)
daily, removing detritus, as many of the filaments as I could, and about
half of the accumulated duckweed from the surface.  Most of the plants grew
and bubbled vigorously, but every day more of them had an algae strand or
two.  So, I called the local water treatment plant for a report, which tells
me that they maintain .5 to .6 ppm Phosphate in my tap water.  Hmmm.  I've
stopped the daily changes, and the algae's stopped it's advance.  (Take
heart, New-Tankers :-)

	So... the question.  What to do about water changes?  At .5 ppm P, a 20%
change would bring the whole tank to .1 ppm, assuming the plants have sucked
it all up in the interim.

	Our tap water in Athens, GA, is very soft (barely registers on my cheapie
hardness kit and other locals spawn Apistos in it, untreated) so I'm not
sure I need a big-time R/O system.  One of the filters is a Fluval 404, so I
could fill a chamber with some sort of Phosphate absorbing resin, but a
cursory search at petwhse.com makes it look like I won't have to buy too
much of that to make up the cost of a small R/O unit.
	If R/O, what should I look for in a bare-bones unit (composite membrane?)
	If Resin, which brands have folks had success with?
	Is there a solution I'm missing?
	Is this even worth stressing about?  (I know there are biological controls,
but I'd like to work my way towards a biotope-ish tank for the Apistos,
which means no snails from Madagascar or Flag Fish from Florida.  I guess
these would be no more of an affront to my sensibilities in the short term
than the Aponogetons, but it is a goal...)

Thanks,
Duncan