Hello JHazzardB,
Well first I'm not familiar with using the
nutrients you are using, and/or if your mixing them correctly. If I were you I would e-mail the company and ask them their mixing directions for growing tomatoes to be sure (unless it's listed on the container or online). I's possible their not designed (or mixed correctly) for continuously fruiting plants, and are unbalanced or contain too much nitrogen.
Also it looks like your growing 8 plants, and with 12.5 gallons of
nutrient solution, that comes out to 1.5 gallons per plant. Your plants don't seem to be drinking to much right now, and that's probably mostly due to your low temperatures. Even so, as the plants get bigger and temps get warmer, they will continue to drink more. The minimum recommended water volume for large plants like tomatoes is 2.5 gallons per plant, and I would double that to 5 gallons per plant. That would be 40 gallons for tomato 8 plants. The larger water volume reduces stress from nutrient concentration fluctuations.
That leads me to another concern. First I wouldn't add full strength nutrient to the reservoir to replace the water the plants drink up. I would either just add fresh water, or if it has been at least a week or more, I would only add a diluted (25-50%) strength back.
Also While going three weeks between nutrient changes should be fine when the plants were half that size, I wouldn't let it go more than 2 weeks at that size, especially with such a small water volume (12.5 gallons). Even with a 40 gallon reservoir, I would probably still change it weekly when the plants get more than another couple of feet bigger.
For now I would probably assume the blossom drop (before they open) is either due to inefficient light, or stress from nutrient solution balance and/or concentrations. Your using HID lighting, and though what you have and how it's placed may be insufficient when the plants get bigger and the top foliage shades the leaves underneath (as long as each bulb is at least 150 watts, 300 total). So that leads me to suspect the nutrient solution first. As I mentioned I would contact the manufacture and verify you are mixing them correctly for continuously fruiting plants like tomato's. Then I would mix a fresh batch each week and fill your current reservoir to 18 gallons. Meanwhile making sure to get a larger reservoir before the plants get much bigger.
P.S.
If you can I would try and get the humidity up as well (into the 50-70% range), and supply some air movement with fans if your not already.