Ok, this one's a little long but I swear it's worth it.
I hear ya man, I had the same issues with my last grow because I didn't have a pH pen, I decided to get a generic one off ebay from china for like $7 or $8 that looks identical to the milwaukee ones that are more expensive, and it seems to do a pretty decent job of it. I just look at it as an investment, and it makes things soooo much easier. don't forget calibration fluid to make sure the pen is accurate or else that could cause other problems. if you are using strips or color test kit you should still be able to gain some progress.
That's ok that you can't change the soil mixture, I always feel like I want to change the soil mix after encountering problems, but it just makes me that much wiser the next time around. Also, sometimes adding nutrients alone to the water can be enough to lower pH, depending on the nutrients.
As far as lowering the pH, I would start with trying to find what the pH is to begin with. If you use straight tap water, your local municipality should have water reports posted online, and if a recent one isn't up they will email you one as long as you search in the right place. It is required by law for them to have these reports, and it is very common for many different people ask for the quality report (after all, it is supposed to be potable), so asking will not draw any attention or raise any eyebrows. If you use bottled water, like I do, then water reports can be found online for almost every company out there. These reports should state the pH or at least an average range it will fall into. Find that number first, so you know where to start. I would say at the least it is difficult to estimate where the pH will end up when trying to adjust without a meter, but in this case a little more than necessary won't hurt it too bad, especially if you're up around 7.5, and they say the soil itself can be .2 to .3 higher than the runoff we read.. so keep this in mind also.
Now as far as actually lowering the water we are about to feed her, you can use a few things that are relatively cheap and everywhere. Yes, you can use vinegar, but I have heard double-sided claims on which type to use. Some growers have found that "white" or "distilled" vinegar gives them more accurate readings and maintains the pH longer than "apple cider" vinegar, while others have found the opposite true. I personally think it depends on your soil mix. You can also use lemon juice that comes in big bottles for like a buck or two at the grocery store, just make sure it's the concentrated juice and not lemonade for drinking.
I have heard that lemon juice tends to keep the pH lower for longer than vinegar, but have yet to compare and really depend on the stuff.
You should only have to use 1 or 2 teaspoons of either to lower the pH to an acceptable level for watering, but again you will find differences in how they each react to your water and depending on the brand, so less or more could be needed. Personally, if I had to choose, I would try lemon juice first (seems most natural), and if things weren't really improving then I'd take a stab at the distilled vinegar (cleaner than apple cider or balsamic). You might even be able to find somebody online who starts at the same pH and lowers using one of these, that way you can guesstimate how much to add per gallon. Some will tell you to not use either of these,
ONLY use sulfuric, or phosphoric acids on soils stating that common acids such as acetic acid (vinegar) are toxic to plants, but many swear by vinegar and lemon juice all over the forums and use them not just for emergencies but all the time. I won't really get into the sulfuric and phosphoric acid, you can simply google it if you wish to make a pH down solution by using them in very diluted forms. Phosphoric acid is car battery acid and is relatively cheap in concentrate and should be sold at almost any auto parts store... so that is another alternative because pH down sold for growing is phosphoric acid. It is purely safe for your plants as long as it is the correct amounts mixed with water, and technically should do a better job at keeping the pH down over time versus the lemon juice or vinegar. I have heard some say the pH is up the next day after watering with lemon juice or vinegar adjusted water.
Well, shit it's late and I've been searching other places trying to make my answer as legit as possible because I know how frustrating this kinda stuff can be, so I hope I helped you out some and good luck with your search, and don't forget to keep us updated! time to smoke a bowl and pass out,
peace!
