TDS meter chart

I use one. I was shocked to find how low the levels were with AG's recommended feeding for Tomatoes and (Sweet) Peppers. It was reading at about 1/3 of the recommended levels. For Hot Peppers, it was catastrophically low. After I tweaked the feeding amount and adding CALiMAGic, I saw much better health and productivity in my plants. I recently switched to all General Hydroponics products, and am very pleased with the results, across the board. I think the AG Nutes are fine for Herbs and probably for leafy veggies, but lacking for heavy fruit producers. This is all IMHO for "my" plants, and YMMV. Everyone has their own opinions and preferences.

This chart is great, as is the whole site for info ph/ TDS/PPM Levels for Vegetables

If you live in an area where your ambient temperatures have wide variations and/or high temps that will affect your water, you might want a 3-in-1 with temp. My water temps don't vary so much that it was an issue for me, so I use the HM Digital TDS-EZ

It came accurately pre-calibrated, and has a manual calibration screw to use with 342 ppm NaCl calibration solution, if it needs tweaking over time. That's a nice plus, as many of the reasonably priced units do not. It was excellent from the start, and I love it.

That's outside my hydroponic wheelhouse. I've only grown leafy veggies in my garden. Hopefully someone else can answer that, or there's always Google.

I'm growing kale now- I'm doing 5ml each of the General Hydroponics every two weeks and they look happy, same as my lettuce gets. I don't know if they would grow faster with a higher concentration though. Larger plants like cucs get 5ml each every week.