My system is already 6 weeks old. I have had the aquarium established since last year. i am growing in my basement apartment currently using a fluorescent grow light. The light is pretty close to the grow bed. I have not had very much plant growth since I started putting plants in my system. I have a mint plant, and some basil sprouts which just seem to stay sprouts. My guess is the water quality. My nitrate is about 150 ppm and my ammonia is about .25 and nitrite zero. My pH however is high, 8.2. i was told earlier not to mess with my pH. My water is pretty hard, and i have tested the tapwater showing it basic. Could there be a correlation with my high pH and plant growth? I would have at least expected mint to grow a little faster than it has. it doesn't seem like it's moved at all since I put it into the system 3 weeks ago.
be careful with mint in an aquaponics system, they can take over. by the end of last year my mint was growing into the plumbing and plugging up the pipes...later I watched one of Murray Hullam's videos, "Let me tell you about planting mint in an aquaponics system...DON'T plant mint in an aquaponics system."
Funny, I planted mint in my system when I first started too! And well... It certainly took over!! The mint grew like crazy while everything else pretty much died. But to answer your question about pH, from my experience at least. I have had a constant pH of 8.2-8 over the past three months, and nothing has done well except for the mint. Nitrates just FINALLY showed up a few days ago (after I ripped out the mint plant). I have had consistent readings of 0 Nitrites and 0-0.25ppm Ammonia.
So all that being said, I would expect that at least your mint would be doing well...
Also, I have heard that the nitrification process is supposed to reduce pH over time.
What kind of light is it (T-12, T-5, T5HO, Compact fluorescent, etc) and how far away from the plants? Just saying "pretty close" doesn't really mean much.