Please help - Brown Spots

Hi so I am pretty new to growing, and this is my first grow.

I noticed these brown spots appearing on one of my plants today. It looks like the leaves are curling inwards and drying up. When I touched it was almost crispy to touch. I think it is because I lowered the PH a bit too much... it was sitting at around 4.7 overnight. Since then I bumped it back up to 5.5 Hopefully this will help.

PH 5 ~ 6.0
PPM 150
18/6 LC

My other 4 plants are doing just fine.

Any help will be awesome thanks.

Looks a bit underfed but might be your ph, I would wait to see if it responds to raising your ph and if that doesn't work then I would bump up your feeding a bit. What's your waters starting ppm?

but if your ph was only at that level for a short period of time then I would say it's not your ph

Thanks max
I was planning on bumping feeding during the next res change which will be very soon. I am going to see how it reacts to the PH change and go from there. I am pretty sure it is just due to the drastic change in PH. From some pics ive seen online it looks very similar to what I'm dealing with.

The odd thing is that my other 4 plants are doing just fine. Other than the small one recovering from a water pump malfunction, they are progressing everyday. The big one is already starting to smell skunky.

The only thing about that is it wouldn't cause the leaf to look like that, that quick. Did it take time to come to look like this or did it happen in a short period of time. And about ppm's, if your using tap water you might wanna test it before adding your food to see what it is. Say your starting ppm is 120 then your actually only feeding them 20 ppm of food.... good luck dude. BTW what food are you using? Organics are alot weaker then chemical

Nah it pretty much happened overnight. I didnt change anything last night except adjust the ph down.
I was using filtered water that has less than 10ppm. But now I am switching to tap water, which is less than 50ppm here. So when I last checked the PPM was at 180.

The nutes I am using are Botanicare line. I added nutes about a week and a half ago, and I gave it a real weak 1/8 the strength. When I do my res change tomorrow I will bump it up to 1/4 the recommended strength, depending on how the plant reacts to the PH being fixed.

then it def could be ph issue

I use two meters, ones a Hamilton PH pen which is brand new and calibrated and the other is a old Nutradip tri meter which is old but calibrated and working well. So my PH readings are accurate... (I hope)

It definately is the PH because I checked on them this morning and they haven't gotten any worse, but the signs of damage are still visible.