No string of O2 bubbles.

My planted aquarium has been in operation for a year but I've never observed
any srting of 02 bubbles streaming from the plant. I've in fact achieved
lush plant growth with MV readings in excess of 500.My tank setup is as follows:

        - 500 litre, trickle filter (overflow)
        - 150 W metal Halide
        - C02 injected. Continuous PH monitoring
        - temp maintained at 28 degrees celsius, general hardness at 4
degrees german hardness, carbonate hardness at 3dh

 My questions are:

1.      Is this normal? I've been reading postings from this list and
everyone seems to be achieving this phenomenon except me. Is it because my
water has a slight current as there is a powerhead swirling inside my tank?
Plant growth is normal and in fact my cabomba is growing well.

2.      As my tank was converted from a marine setup, I had the chance to
start everything anew with some equipment carried forward. Right now, my
ozoniser and protein skimmer are idling. Can I use them in my feshwater
plant setup? What does the 500 plus MV reading which would be unattainable
for Marine Tanks mean for me?

Any replys are much appreciated.