Mystery Tank

Hi all, hopefully you all can shed some light on this mystery. We have a 52g
tank set up over a year. Two weeks ago we noticed that the co2 level was
50ppm. Quite high we thought so we cut off the DIY yeast engine. We are doing
weekly  15% water changes with DI water enhanced with trace
elements(Electro-Right)and pH adjuster. The co2 has always been in the 5ppm
range. This week we also added PMDD enstead of the Kent fertilizer we were
using. Yesterday we got the following:

CO2	>100ppm
Fe       1.5ppm
NO2	0
NO3      0
Nh3	0
pH	     6.7
dGH	4
dKH	5
Temp 79f
We measured the co2 with both a LaMotte and a Tetra test kit.
Can anyone suggest what is going on with the CO2. The bioload consists of the
Echinodorus Quadricostatus			(dozens)
Echinodorus Osiris Rataj					( 1)
Barclaya Longifolia									(1)				About to bloom
Vallisnaria Spiralis									(8)
Sagittaria Subulata									 (6)		In bloom
Hygrophilia Difformis               (2) Large

Hygrophila  Polysperma            (2) Large
Ludwigia Repens                       (12) Large 2  plantings one blooming
Cryptocoryne Cordata               (1 ) small
Cryptocoryne Wendtel??           (1) medium
Alternanthera Reineckii            (6) not doing well

Cardinal Tetra                            (2) 

Neon Tetra                                  (1)
Gyrinocheilus aymonieri             (1)
SAE 	                                       (1)

TIA  Ric