Cal/mag AND spider mites?

I sure hope not.
Day 18 flower. I saw what I thought was fading after the first week of flower. I upped the veg nutes slightly and skipped a watering as I usually feed-water-feed-etc.
I'm using botanicare pbp grow (advanced chart) with liquid karma. I haven't started the bloom nutes yet. I thought I had early fading on my last grow so I've kept the veg nutes into the third week of flower.
I'm using Toronto city tap water.
The other plants in the tent have the same stems but the leaves are unaffected.
I have looked all over for spider mite damage but I can't find anything. Nothing on the leaves' back nothing where the petiole meets the leaf.
Thanks in advance growers.

Looks like there is crap under the leaves. Hard to tell with that one picture Those yellow spots under leaf could be eggs.

Keep looking under the leaves.

The top of the green leaves will start to look like they are being spray painted yellow with yellow dots that get bigger over time. This is the mite piercing the underside of the leaf and sucking out the juice.

have a look on the under sides of the leaves for mites with a 30x loup , but it looks like cal/mag def
what are you ph ing at, do you know the EC of your plain tap water ?

I agreed with @bobtokes

@bobtokes hey bob I believe you were correct. I've never tested my tap water because I was told I didn't have to. I was also told that I didn't need to use cal/mag. Whatever. Half way through my second grow i think we've figured it out. She's had one dose and she's already looking better. All the purple petioles on all the plants are starting to green up.
I've been also looking for mite evidence but haven't found anything. I'm keeping my fingers crossed.

just spray your next lot some time during veg better safe than sorry

I set of a fogger bomb once every month to 2 months just to kill any bugs in the grow area