Unfortunately testing pH doesn't tell you how much baking soda to add, you really need to test KH to tell. If you test for KH and keep it in the 100-200 ppm range you never have to test pH because you'll know it's 8.3 to 8.5. On the other hand if you only test pH and it's 8.5 you'll have no idea if KH is 200 ppm or 2,000 ppm. You'll only know when pH drops and then you're already in trouble.
However, just as an estimate, assuming the KH is 80 ppm (I assume it's even lower) you could add
2 to 3 kg of baking soda to get to get into the 150 to 220 ppm range. A little over 200 ppm would be OK. So you might want to buy a sack at a restaurant supply. It lasts forever if kept dry and if your water is that acid you'll probably need more.
There is one danger you should understand before tossing in any baking soda...if you don't test ammonia you won't know if it's zero. If it is above zero, which isn't good by itself, increasing KH can make a bad situation worst as higher pH makes ammonia more toxic. And unfortunately a lot of people think if you add baking soda slowly it will change pH slowly and that's not really very true. You could add 1 kg and not see much change in pH, or see a short term pop and have it go low the next day. When KH builds to a buffering level pH will shoot up. The fish can handle the jump up as long as there's no ammonia. In a perfect world you would raise the pH slowly, but that's not very easy to do.
Here's a
good source if you want more info or a more expert opinion. There are a lot of sites that just kind of copy what they read on some other site and each translation loses some info and adds some info so not exactly accurate. The higher end Koi keepers take this stuff pretty seriously.