Please give me a quick up to speed on PH

I am almost sure that the problems I am having is due to PH. I did not pay attention to the level thinking the water was fine. I have flushed the plants with adjusted ph water at 5.5 and the water coming out still tests 7 maybe a bit over. I am going to town and looking for a good ph meter to be more exact and some ph up and down solution. I have read that plants have been reported to do fine from 6.3 to 6.5 and some do well as low as 5 But from what I have read 7 and higher is going to lock out nutes. So to fix this am I correct in flushing with corrected water untill I get a waste water coming out of the drain holes at 6.3 to 6.5? then continue to use corrected water and feeding solution? If so how long does it take to see the results of corrected ph? 1 day 2 days? I think I have been fighting this since the beginning. That is why some of my plants are 1.5 months old and only 3 inches tall and dying. any help appreciated. thanks in advance.

make sure you test the ph after the nutes,i used town water with a ph of 8,and the nutes brought it down to 7,i used this for a year with out problems or a meter,but sometimes differant strains react differntly.but need alot more info to help,like nutes,soil ect ect.

I have 5 plants. two are in the same pot and are about 2 and 1/2 months old and about 17 inches tall. it was started and repotted in plain potting soil from Wal-Mart without nutes in the soil, this one has not had much in the way of nutes, I used MG and it was to strong burnt it and flushed, she recovered nice but then when I changed from Flor. to HPS the light shocked the plant. I moved the light high above the plant and she recovered, now I have brown spots on the leaves and yellowing on the uppers leaves. This is the same with all the plants.
, one was started in Miracle Grow seed starter and repotted in plan potting soil, This one is about 1 and 1.2 month old and has not grown fast. it is about 8 inches tall and is now yellow on bottom leaves and yellow and green on top leaves, brown spots are starting to appear.
One was started in miracle grow seed starter and has not yet been repotted. It is about 10 inches tall and seems to be doing the healthiest out of the bunch but now is starting to wilt. I am going to repot it tonight in a bigger pot and in potting soil. This plant also was in the most seed starter with the mg nutes in the soil I wonder if the nutes off set the alkali in the water for a while with this plant? The fifth one has not done well at all it is 1 and a half months old and is only 2 inches tall. the whole plant is yellow with only a small amount of green left. it was planted in a 3 inch peat pot filled with MG seed starter. I have noticed that the water I was giving them was from a deep spring that was heat pushed up in a type of geyser, I noticed my coffee pot getting a lot of lime on the inside. This may be the problem. I have just today started to use my britta filter to clean tap water for watering and metering it with the ph tester and bringing it to 6. I went to 4 nurseries and lowes and no one had any ph up or down. I am going to a bigger city Saturday and see what I can find there. Any help appreciated, and if you need any info I forgot to add please do not hesitate to ask.

what is the ppm of your water you start with,mine was 900+ before nutes,so i needed an RO machine,a brita filter doesnt do alot depending on the quality of your water,your best to find that out so you can eliminate water as a cause.