Tap water is often slightly basic to reduce corrosion and leaching. You absolutely need to correct to a pH that is tolerable for plants. Put some of this in a jug and pH adjust the jug such that you’re adding water slightly more acidic than the tank. Do this every hour or so (so as not to shock the fish) until you’re in range.
You will need to often babysit this as you add water to the system, as the tank is likely buffering to too high a pH as well.
General Hydroponics HGC722120 Liquid Premium Buffering For pH Stability, Quart, Orange https://www.amazon.com/dp/B00CJID1I4/ref=cm_sw_r_cp_api_i_VrffFb14GXQ04
There's a liquid solution called pH Down that exists specifically for this purpose. I've used it occasionally for plants that are really sensitive and it seems to do the trick, although a little goes a long way.
Why dont you just go buy some pH down (phosphoric acid).
Its only $13 and youll only need like 300ml.
I slowly added it while I had my fish in my 120Gal system and they tolerated it well.
A little goes a long way. Get a smaller bottle. Or use a lemon-works just as well.
7-7.5 is pretty high man you are gonna need some PH down i have to use it every watering where i live i get constant 7.5 ph and that is not good for marijuana and can stunt them from what i read 6.0-6.9 is the perfect ph area for growing in soil (for coco it is different)
here is a link for info https://www.royalqueenseeds.com/blog-the-perfect-ph-value-for-a-cannabis-plant-n87
and here is a link for ph down https://www.amazon.com/General-Hydroponics-Liquid-Fertilizer-1-Quart/dp/B00CJID1I4/ref=sr_1_6?dchild=1&keywords=ph+down&qid=1602186076&sr=8-6
https://www.amazon.com/dp/B00CJID1I4/ref=cm_sw_r_cp_apa_i_EVDhFbSA8HET7
Depending on what the powdered stuff is made of it can degrade quicker than others causing pH to bounce back
Can I use this for ph down? It says made from food grade lactic acid. The reason I ask is because my son used to work at the hydroponics store and we have a few gallons of it.
https://www.amazon.com/General-Hydroponics-Liquid-Fertilizer-1-Quart/dp/B00CJID1I4?th=1
pH Down: https://www.amazon.com/dp/B00CJID1I4
Pipettes for said pH Down: https://www.amazon.com/dp/B085VXMY78
Fancy pH meter: https://www.amazon.com/dp/B01ENFOIQE
Cheapest one I could find on Amazon. I did consider a few other brands, but the GH bottles had the best reviews.
Adding an acid or base to adjust pH via the "change the water" approach is the most common and typically the most tangible solution (huehue) to pH problems. It's also super reliable because you know exactly what's going into your plants' rootzone. The tradeoff is simply that you have to monitor the water's pH. I do this during the winter with my indoor tropicals using a common hydroponics product called "pH down".
But let me throw another thing at you since SFGate mentioned it and you don't seem scared of chemistry.
If you'd prefer to take a more hands-off approach to pH adjustment, you can try elemental sulfur for an immediate adjustment or a Soil Acidifier for an immediate change and long-term effect. Since you won't know to what the pH the water is being adjusted, there's a bit of a learning curve here. That said, a sprinkle of this on top of the soil typically does the trick for my outdoor pots.
I'd avoid Nitric acid at all costs for home use.