I have chronic low nitrates, frequently zero. However when working on an Algae issue I found I had high phosphate levels. I corrected that by adding a nitrogen fertilizer. When I did my PH went up. The nitrogen fertilizer contained potassium nitrate. I suspect the plants used the nitrate and left the potassium behind because they didn't need it. Potassium can have a strong impact on PH and alkalinity. I have seen PH levels as high as yours when using potassium containing nitrogen fertilizer.
In your case I suspect the tap water is high in potassium. It also might be high in nitrates, many people have this issue. I suspect your high nitrate levels and the growth of your plants may have depleted the tank of phosphates or other minerals. A deficiency in any nutrient will plant growth will be slow or temporarily stop plant growth. As a consequence your potassium levels may now be high. You can try to correct this by doing more frequent and larger water changes. Most people do remove and discard about 50% of the water once per week If you are not doing that you should start. If you are doing water changes do more, more frequently. over time that should get the levels down.
If you can determine with a test which nutrient is in short supply you could correct it with a fertilizer. II would not recommend you use phosphate fertilizer without first measuring them. Most phosphate fertilizers contain either potassium or sodium and those can increase your PH levels. if your phosphate levels are ok you might have to use a micro or trace fertilizer. Preferably one without sodium or potassium.
