What is your water testing at?
Also what's your lighting schedule?
Do a water change weekly (10%), and have your lights on for only 8 hours in the day. Also if your tank is buy a window with real sunlight coming in, you might want to move it.
Also if you haven't already, don't use tap water! Go to the local fish store and buy RO
water. This will help with your issue.
Maybe some snails if you can. Also you how much flow is in your tank? You need some power heads to get the water moving around!
From the look of your tank, I would try and get the rock more in the center and try opening it up more with some more openings for your fish to swim though and so the rock is not touching the glass in the back, front or sides. Get one Nano Power Head and put it in the back of your tank to get some flow going behind your rocks, and maybe another nano power head in the front on the opposite side! Flow in your tank is important, the rock is there for filtration and to give your fish a place to rest outside of the flow at night. Good luck!