Salt can cause ionic stress to the plant, which can inhibit metabolic processes such as protein synthesis

Salt can accumulate in older leaves, which then kills them, reducing photosynthesis capacity.

Now thats for the geeky part. Reality says that low level can be disregarded, but if youre going for a light feed regimen, try to put a little less of whatever contains nacl...

As a comparison, sea water total dissolved salts stand at 35000ppm.
A date tree can take up to 12000ppm of salinity.

Ive been growing hydro from tap water forever and never had problems. Is RO water at 0 ppm better to start with? Of course it is...is it worth the trouble and expense of an RO system? Not in my opinion...

I can't say definitively but if it's worked previously, it shouldn't be a problem now. you should have you water tested to determine how much of what nutrients are already present. If you have a lot of iron and calcium already, you needn't be too concerned with adding them.
Also, recognize the limitations of a TDS meter. if you take water and add a dissolved salt into it until the meter reads 500 ppm, that doesn't mean there are 500 ppm of dissolved salts in the water. It means there are simply enough dissolved salts to make the meter read that way. TDS is simply a conversion of EC. to be useful you need a baseline measurement of your plain water (which you have) then calculate the amount of nutrients in the solution you're going to make and measure that. whether it matches the meter of not is irrelevant. you're developing a relative scale.
Oh! and 500 ppm is not a "weak" nutrient solution. That's about what I use to grow sweet peppers.

Thanks, MisterK and grizzman.

The TDS meter has 3 conversion scales, and I've selected "NaCl" which will hopefully give me fairly accurate readings at least for my baseline reading. From there, I guess calculations are needed to supply accurate values for the fertilizer solution. BTW, my standard "weak" solution read around 300 ppm, and 500 ppm is the most concentrated that I expect to use.

Too bad sodium isn't any better for plants than it is for humans.

You could switch to Potassium Chloride pellets for your water softener if you are worried about sodium.

Get an RO filter, 0 PPM water. It's the best way to go and plants love it, plus it makes it easy to dial in your nutes.

RO also wastes a ton of water, some systems create almost 2 gallons of waste water for every 1 gallon of RO they produce... So, not only do the filters cost money, but your water bill will also go up a ton...

"RO also wastes a ton of water, some systems create almost 2 gallons of waste water for every 1 gallon of RO they produce... So, not only do the filters cost money, but your water bill will also go up a ton..."

It does waste some water. There is a tube for clean RO water and a tube for waste water. I would say there are 2 parts output for dirty water for every 1 part of clean water. So Yah, Wasting 2 gallons per every 1 ballon or somewhere thearbouts. This was can easily be collected and used to water soil plants or other things though if you want.

For me, I use a 55 gallon drum Ebb&Grow system and I only change the rez every 2 weeks, so I am only using ~100 - ~150 gallons every 2 weeks. That sounds like a lot, but it's like taking a few baths in the bathtub. My water bill hasn't increased very noticeably. I had 400PPM water in my tap and the RO made a big difference.