Thank you for the replies!
To further complicate things, I had my water tested at Petsmart yesterday - and everything was great except for the pH that tested at 8.3. I'd tested my water beforehand at home with my api master test and got the same results I posted here - so I'm not sure which test was inaccurate.
I did order ph testing strips that will be here tomorrow to see how that compares. I also ordered fluval peat pellets to introduce slowly to see if that helps.
I'd be much happier if my pH was closer to 7.6 - I'm guessing my fish would be happier too, especially since I'd like to add more fish in the future. My current fish are probably going to start aging out soon. (I mostly have five year old gold tetras, neon tetras and corydora - they've survived three aquarium changes, and then this apparently higher ph water several months ago when I moved to a new house, haha.) I also have several nerite snails, though I lost all of my older snails when I first moved here and had trouble introducing newer ones - I guess because of the higher ph.
Would a ph of 8.3 explain why I've had a hard time keeping plants alive? I'm using eco-complete for the substrate, have a 24/7 planted plus light in a 29 gallon, homemade fert capsules, and dose with co2. I'm FINALLY seeing slightly better results lately, but if lowering the pH would help, that would be even nicer!
...or maybe I should just get cichlids after my current stock dies out, haha.