Rust spots on leaves - Hydroponic

Hello! Any help with these rust spots on my leaves would be greatly appreciated. Older foliage is in really bad shape, newer growth shows some signs of the spots.


The deets...
Afghani and mystery indica strain are both affected.
I’m now 52 days into the grow and have been trying to problem solve this thing ever since it started about three weeks into the grow.
Hydroponic system
RO water
Using all nutrients that are a part of Flora Series Simple Drain To Waste, replacing water every 7-10 days.
Top of plants temp range 70-83 (usually not above 80 though)
PH 5.8-6.2
I periodically spray with neem oil as a pest preventative and am now using a no-pest strip for pest preventative to avoid neem oil on budding plant.

Looking at pictures online this seems to be either nutrient burn or Mg deficiency. But here’s what is confusing me…
I first noticed this at three weeks into the grow. The day before the problem the ppm was at 610. The next day the ppm had dropped to 490 and I started noticing these rust spots. The plants drinking up the nutes enough for the decrease in ppm had me thinking that it was not due to nute-burn. However, the next morning the ppm was back up to 700. So, I have no idea what was going on.

More recently, the ppm has stayed more consistent. I have gradually raised the ppm with each res change to accommodate the growing plants. Between res changes the ppm stays about the same, making me think that since it is not getting higher as the plants drink water that I am not setting it too high to begin with.
Right now I am four weeks into flowering and the ppm is at 890.
Could this be nute-burn? Or something else? It looks like nute-burn, but I’ve read that the sign that the ppm is too high is if it goes up because the plant is using more water than the nutrients, and this is not the case.
Thanks for any suggestions!!

they look low on phosphorus and potassium, micro nutes

it is not nute burn. add more of everything, hydro you need to be placing ALL nutes in there manually, from day 1 to shortly before harvest. and babies still need P, K, mo, mg, ca, fe, copper................

make sure your bottles of nutes have a wide variety of stuff. i never go with just 1 company´s products.

check the reservoir for salt build up too, that could be a sign of some salt toxicity. make sure your area is clean, or you might run into unexpected overdoses on nutes. that happened to me the first time i did DWC and it sucked!

looks like calcium AND magnesium deficiency to me,,, but hard to tell.

lower leaves falling off? getting crispy? cal and mag.

Thank you so much for your responses.

Regarding phosphorous, potassium, calcium, or magnesium deficiency, the General Hydroponics nutrients seem to do a good job of providing good ratios of this stuff. But what do I know, this is only my second grow lol. Below is how much nutes I added to 4.25 gal of RO water a few days ago during my last reservoir change, along with the ratio of the elements in question…

The thing that perplexes me regarding if it is a nutrient deficiency, is that the General Hydroponics Flora Series Simple Drain To Waste feeding chart recommends no more than 700 ppm during this stage of growth. Not only is my ppm higher than this (916 as of this morning), but most things I read say that marijuana ppm should usually be lower than the recommended ppm.

Regarding possible salt build-up, I do get quite a bit of salt that accumulates on the lids that I use to cover the empty reservoir holes. But I do try to keep the area fairly clean and change the res every 7-10 days, so hopefully this is not the issue.

And yes ‘Guerilla Gardener’, the lower leaves are definitely the worst and are getting crispy with some falling off.

Thank you for your suggestions! This is definitely giving me some things to think about. If you can think of anything else please let me know.

yes it is cal-mag deficiency, if you are using RO water , you need to add cal-mag every feeding. GH now has a cal-mag called cali-magic if you prefer to stick with their line, if not i recommend using pro-cal by green planet.you will notice a change with your plants in about a week after adding cal-mag to you plants, i promise and no you definitely have no nute burn. its just the cal-mag u are lacking.

Thanks, you guys are awesome . I will look into getting me some of that CALiMAGic from GH and see what happens.