Hmm, you're definitely pHing correctly so that shouldn't be a problem. So you are pHing your water to 6.5 with a digital meter and then checking your soil with another meter? Same meter? (Some can test both soil and water but unless it was over 150-200$ then it probably is only made for one or the other). I'm trying to figure out what is taking it so low, did you add something to your soil like peat moss? Mulchy soils with a lot of bark can also be acidic.
That's strange that you are pHing your water to 6.5 and it is testing so low in the soil (5.5). Normally soil will buffer pH a little bit, I just use the indicator die so I'm probably a half a point off when I pH but I still don't have any problems. Granted I have lime, that helps.
If your pH in your soil is actually 5.5 then that could very well be your problem. But honestly it kind of looks like overfeeding, which would be surprising considering your nute schedule.
And also, there are probably a number of bacterial or fungal things that could cause problems coming from the fish tank water, but that's all a possibility and a guess. I have 2 five gallon buckets that I Keep filled with tap water, the chlorine dissipates if you let it sit for 24 hours and mine sits for four days usually. Also, spending a dollar a gallon for some distilled water might be a good idea if you don't trust the chlorine thing. 90% of the time when people on here are having problems, flushing with pH'd water is what solves their problems. I would try using a different water source to see if the water is your problem, BC honestly it sounds like your doing everything eight for the most part and that's the only out of the ordinary variable that I'm noticing.
Who knows; fish produce nitrogen, which is not a good idea to give to your plant during flowering (at least not as much as you give it in veg).
The more I think about it, you really should go spend 5$ on 5 gallons of distilled water and try that out. You still have a chance to save some of the quality on that plant if you can stop it now.
Either way man, just don't let this bad experience get you down. If this grow doesn't pan out, get some bag seed or order more and try again. Remember, keeping everything as simple as you can will work better than anything. Less is more with this plant. I'm a first time grower myself and have done tons of research, had almost every problem in the book starting out but I got it dialed in finally and couldn't be happier.
I'm still vegging mine until incan buy my HPS this weekend, but I went through a period where I thought I was going to lose them all. The more I did to fix them, the more problems I had. I bought a bag of fox farms ocean forest soil, and added a couple teaspoons of dolomite lime, water every 4-5 days and they have started exploding.