Vlad walked me through some of the numbers and what to add, I wasn't really counting on growing tomato's but it did work in the end, here's a link: http://community.theaquaponicsource.com/forum/topics/measuring-stuff
I use to have the same problem. Recently I read that if you want flowers and fruit that you need your nitrate levels to be low but if you want leafy green plants then they need to have a high nitrate level. My nitrate is so low now that it is almost unreadable after three years and in a five thousand gallon tank with what we've been told have a LOT of fish. We produce lots of papaya, tomatoes, eggplants, coffee, guava, lemons, etc. Our basil is nice too along with swiss chard. In all we about 100 different variety of plants but I've never done well with lettuce.