Rust colored spots - nute burn or PH?

[Uncle T]

Help!!!
I am growing several strains. I have a Humboldt, Silver Haze, and an unknown "Nor Cal" strain.
All started from seeds.
Outdoor grow.
Grown in Fox Farms soil mixed with perlite.
The plants are about 7 weeks old.
They are in 5 gallon pots (very doubtful they are rootbound)
I use city tap water. The local hydroponic shop owner said the city water was great as far as PH, and the soil acts as a buffer. I suppose I should get a PH tester and test for myself.
I water about every day, depending on the heat. I wait until the top dries out a little first.
No bugs that I've noticed.
I haven't needed to fertilize yet.

OK, here's my problem. I have several plants that are growing perfectly, the humboldt strains. The other 2 are having problems. From what I've read it looks and acts like nute burn, specifically Nitrogen. Last year I overfertilized and had rust coloring on the leaves and flushed them well, and it resolved. So this year everything was going well, and all of the sudden I started seeing rust spotting on the fan leaves, not the lower ones, but midway on the plant. If I let it go, the rust coloring turns yellow, then that part of the leaf dies. If I flush it once or twice real well, it stops the yellowing and dying process in about 2 days, then it's ok.
I showed the plant to the local hydo store owner and he said it's not he PH, but that it needs nutes. (I think he just wanted to sell me expensive fertilizer) I didn't agree with that and went home and flushed it instead. I figured if it was nute burn, adding more nutes would only make it worse, so I went with my gut feeling of it being over fertilized and flushed it. That plant is doing fine, but now another is doing the same thing. Strange thing is none of the Humbolt plants are doing this. They aren't even showing the slightest bit of needing ferts yet. They are very lush and green and growing great. From my experience and from what I've read, lack of nitrogen in diagnosed by a uniform yellowing of the lower leaves, starting at the bottom of the plant. This isn't what is happening. And they are growing fast and furious when they aren't being burned.
I talked to the guy at the other local nursery where I buy the Fox Farms soil. He said that several guys have come in and said they are having over-fertilization problems with the Fox Farms soils, so they are mixing it with Happy Frog so it doesn't run so "hot". His term for soil with a lot of nutrients I guess.
So what are your thoughts, does this sound like nut burn? It seems everytime I flush it, the plant gets better in a few days. Is it still possibly a PH issue? And if so, why would it just affect certain strains?
PLEASE HELP!!! I'm a medical patient who cannot afford to buy it, so if my grow isn't successful, I will be misreble.
THANK YOU in advance to anyone who takes the time to help, even if it's just a simple reply.

[piff toker]

Dude, it may seem that I am having the same problem. Today, I flushed my plants, and hopefully I look to see a change. My research says it could be a PH problem, chemical burn and/or overwatering. I am a first time grower and am still researching. Good luck and if I find any information, I will pass it on to you.

[Uncle T]

I just got a PH test kit and tested the city water I'm using to water the plants with. It's more alkaline than the test kit will show. It goes up to 8.0, which is greenish, but my sample was blue, so it's got to be around 8.5. So does anyone think this is the problem?
If so, how can I lower the PH of the water I am using to water the plants with?
The guy at the hydro store has "PH Down" product, but that seems like it would get expensive to have to but an additive for my water.
I thought I read somewhere you can add vinegar to make it less alkaline. Is this correct, or internet ridiculousness?
I still suspet nute burn, but a high PH can't be helping.
Any ideas?

[piff toker]

i use vinagar to bring down my ph as i said before this is my first grow and dont know the affects of this but it seems to work and i havnt read anything harmful about it

[piff toker]

also your ph level is way high like mines was after reasearching i found that the ph for hydro grow is 5.0-6.5 and for soil it is 6.5-7.0 i could be wrong but i am pretty sure that is correct

[Uncle T]

Ya, I am going to get some PH lower from the hydro store tomorrow. I looked at a good website. The Ganja Guerilla website for sick plants. Ganja Guerrilla's Hall of Marijuana Plant Abuse
It has some great photos for nutrient deficinecies and overferting. Also I found an alkaline (8.0) PH photo that looks just like my plant's leaves. So I hope that's what it is.
Good site, you should check it out. I hope this corrects some of the problem.
I'm still open to other ideas, comments, and past experiences that people here have.
Thanks!!!!!!