Where is the ammonia in my new planted tank coming from?

I have set up a new planted tank and encountered a problem I've never seen before. The tank is 2 weeks old and has only plants, no fish yet. It is a 15 gallon with an Eheim Ecco 2330 canister filter, Hydor 200 watt in-line heater and a Hydor Nrg Green Co2 system, 2 24 watt T5 bulbs, 6700k. The substrate is 3" of Eco complete. Plants are Rotala Indica, Ludwigia Repens, Dwarf Hairgrass, Micro Chainsword and Dwarf Baby Tears. One piece of Mopani wood, presoaked for 3 months. Water is Arcadia bottled water, pH is 7.2, kH is 6.0, gH is 10. I dose with .8 ML Seachem Flourish twice a week. Phosphates are 0 ppm, nitrite is 0 ppm, nitrate is 5 ppm. The plants are growing well but the problem is ammonia. It's at 6 ppm! I have absolutely no idea where this ammonia is coming from since nothing I've added to the tank should be producing any. Before adding to the tank the water tests 0 ppm for ammonia. Any ideas where this ammonia is coming from?