Hello JHazzardB,
I have some suggestions for you, but I don't think your going to like them.
1.
CHANGE YOUR WATER SUPPLY. Buy a small but good water filtration system. Hopefully something that contains a combination ion as well as carbon filter. Carbon filters alone can't take out dissolved solids. If you can afford it, get a small portable RO system. You should be able to get one for about $80-$100. You'll also have to change up your water fill system a bit. Have a water storage tank above the reservoir with fresh water in it, and have it gravity feed down to the fill valve to refill as needed.
2.
Change the nutrients your using. Put the masterblend and all it's parts aside, as well as the cal-mag for now. Use a different brand of nutrients. Something like GH Flora series or the more cost effective Maxi gro/bloom series. Or the even more cost effective verti-gro F-combo (or larger quantity for best price). Or something in the JRPeters line like the "Jack’s Professional 5-12-26 Hydroponic". But try a complete hydroponic
nutrient from a trusted brand. Don't use anything you need to add Epson salts to.
3.
Switch to a larger reservoir. For 16 tomato plants you should be using a 80+ gallon reservoir. Again Your relying on your EC meter, and you think your nutrient levels are good because you can keep the numbers on that meter relatively constant. But there are two problems you are not taking into account, and your meter cant. The larger reservoir and good quality water fixes those issues.
4.
Pack up the EC meter, and store it in a closet somewhere. At least until you get the hang of being able to do things withough't it. You don't need it, and your relying on it to tell you what to do. Your focused on EC level's, and you shouldn't be. Plants are adaptive and can take a range of nutrient levels. It's the levels of
whats in that water that maters most, and your meter simply can't tell you that at all. The meter cant tell you what level any of the elements are at. As an example, your water supply may be high in calcium (most are), and your nutrients should have plenty of calcium, then you go and add cal-mag as well, your calcium levels may be at toxic levels, and you would never know it by the reading the meter gives you. That goes for any of the 13 essential mineral elements for plant growth, as well as anything else that may be in your water supply. Again one of the problems I referred to above.
5. Unless your desperate, after you make these changes, I would cut off any current developing fruit. That way there wont be any question's later and easier to tell if the new fruit are developing correctly or not. The BER fruit wont be useful to eat anyhow, and takes the plants energy to try and grow.