The nitrifying bacteria makes nitrates that plants love but it lowers the PH. So you are always adjusting it up. Most plants likes it around 7 PH.
When I first setup my aquaponics system I just bought PH adjusting compounds form an aquaponics store.
I knew there was cheaper ways to adjust the PH but I had way too many things I was building to worry about it.
Now I am out of the compounds to adjust PH. It was time to buy cheaper items that still work well.
Builder’s lime is good for adjusting PH I called about 20 places no one had builders lime Quikrete has a lime product Lowe's stocked it at one point but the stores here don’t stock it anymore. I call the Quikrete they said they don’t ship to our area any more.
You can also use hydrated lime that is what I found locally. $7 for 5 pounds. I used two teaspoons and it brought my PM right where it was needed. So this stuff is very powerful. This stuff is Lots cheaper than what I was using no shipping.
I could also get hydrated lime 50 pounds for $10.75 but I just don't need that much lime when I only use teaspoons of it each time.
I bought some Potassium Carbonate off ebay $19 shipped. It good to alternate between calcium and Potassium to adjust PH to give plants what they need.
I have some Oster shells in a bag ( This helps buffer PH ) with water flowing over it to help with PH. This is cheap but very slow acting.
This should be enough compounds to adjust PH for three plus years. I have around 1,000 gallons of water in the whole system.