Wanna get rid of spider mites? Super simple. Buy Horticultural oil. Mix 120ml to 3L of water. Spray the shit out the plant top leaf bottom leaf stem pot floor wall everything. It dries quick. Come back again like 3hrs later. Spray again. Everyday after that light spray the plan for 1 week and you will never have it again.
The other issues you need to rectify. Get a proper pH pen. When you buy one calibrate it right away before you use it. It's easy.
The next thing you need to do since you city water ( I do too) is to buy a airstone, air pump and hose. You will airate the water. Never let water sit still for too long it's bad.
As for your plant issue just needs some cal-mag. Also stop "winging it" your girls and your buds will turn to shit no joke.