Need Help!! Phosphate readings off the chart!

Hi...
I just picked up a new Hagen Nitrate and Phosphate test kit for my
29gal. I just re-started my tank with fresh gravel and new plants and
lights. I have 95watts, and DIY C02. I also completed a 50% water
change yesterday, and another 25% waterchange today, because of a
small thread/ hair algae outbreak. So I wanted to make sure my water
was clean before testing. The tank has been running for 6 weeks,
cycled and has a good bacteria load going for sure. Maintenance is
kept up religiously. Plants were growing for a couple of weeks, but
are showing signs of slowing down, and yellowing.

Readings are as follows: (after water change)
Ammonia: 0ppm
Nitrate: 0-5ppm
Phosphate: 5ppm+
K:??
Nitrite: 0ppm
PH: 7.0
GH: 8deg
Kh: 9deg
c02: ~20ppm
temp:78
Fish load minimal: (4 Cardinals, Pleco, and a Cory)


Problem here is obviously the Phosphate!! I tested my tap water and it
came back zero. Then I did a test in the tank (after the water
change), and it was off the scale?!! I dont get it! Something is
really leaking Phosphate into my tank...and I dont know what? I have
fertilizer spikes under the plants which have Phosphate in them, but
they are way under the gravel and are never disturbed.

Someone told me that Activated Carbon in your filter releases
Phosphate, but I found that hard to believe as Activated Carbon is
supposed to suck up nutrients rather than release them right???

I'm really stuck on this one! Can anyone give a list of possible
sources of Phosphate, that I might not be considering. At present, I
am not dosing any ferts, and havent since start up. The phosphate just
magically appears for no reason!

Thx in advance,
Nick.