For context, the tank that has become my absolute problem child is a planted 10 gal. For equipment I have: AquaClear 20 HOB, Tetra 5-10 gal submersible heater, and Aquarium Co-Op battery backup airpump attached to an airstone. 

Now for my unrelenting issue. I only have a single female betta in it. It's all I want stocking wise, and she's been in it for over a year. The pH though...using liquid and strips both cap at 6.0. I have a ph meter, and it regularly reads a whopping 5.2. Now, my fish has been fairly healthy. My water hardness (only have strips to test it) reads at 300+ mg/L. I have not been able to get her pH to budge. My tap water comes out at almost the same hardness. I would like to get her water more neutral, as she recently scuffed up her side, and I want to add more plants. Nothing lives in her tank other than an anubias, Amazon sword, and some little one I've long forgotten the name of. Duckweed dies. 

I just want to be able to make her tank just a bit better, but I'm completely lost as to how. Part of me still thinks it's her fluval stratum substrate, but I have little evidence to actually go off of. Absolutely any help would be appreciated. I've looked into ro systems, but that's just not something I can do in my current situation.

Also sorry for the water not being topped up. That's a problem that has since been fixed.

Help me problem solve this aquarium.