Having said that have you tested your tap water? As the other person said your pH is pretty high and likely so is your hardness.
You can have hard water and a low PH or you can have a high PH and soft water. There is no consistent PH / GH / KH relationship. IF you are on TAP water then your water could be like Seattles water which is Very soft. If you are on well water you might have hard or soft water . So right now there is no reason to consider RO water.
Your fertilizer l doesn't have calcium or magnesium which plants do need. GH is a measure of calcium and magnesium so if it is low you will need to supplement the with a GH booster.
Plants also need sulfur, chlorine, and nickel. Tap water is chlorinates which generally creates chloride salts in the water which plants prefer. Sulfur is generally not a problem is tap water. Nickel requirements by plants is very low and that plants can get enough from the fish food added to the tank.
While you fertilizer has and copper the amount in the fertilizer is very low and you might be deficient in one of them. Most fertilizers are very low in these nutrients because galvanized or copper water pipes will typically leach some zinc and copper into the water. But with even very soft water you still could run out of it.
Your nitrates are high because your plants cannot grow enough to consume the amount in the water. The nutrient deficiency you have is stopping your plant growth. With your slow to no plant growth you light is enough so I wouldn't worry about it right now. However once plant growth has been restored it might be an issue.
Beyond that it is impossible to say with any certainty what your problem is. If you is not very soft or hard based on GH then you might have to do a lab tast on the tank water to determine the issue. I have used this mail order test and it helped me identify the the deficiency in my tank and I was able to resolve the issue in my tank.
For each element in your fertilizer or mentioned in the replies to your post verify the levels are not zero. A zero level means you are deficient in that element / nutrient. You can sometimes find a single nutrient bottle fertilizer in the stores or you might have to make your own supplement People on this site can guide you on this if needed.