| You haven't really told us anything about your pond, fish, or filters other than the fact that you aren't confident in what you have. For us to give advice it is a bit like a doctor telling a patient how large a bandaid to put on a wound he's never seen.
Fill in the blanks. How large is your pond? What kind of filtration do you have? Do you have bottom drains, skimmers, etc...? How much water is your pump moving? How many fish do you have and how large are they?
As to the general gist of your questions...
Yes, Baking Soda works for raising kh, with no ill effects to Koi health.
Baking Soda will stabilize your ph around 8.3 when properly dosed, so it will not help you lower your ph to 7.5.
Coral, Oyster Shell, Lithaqua, are slow release kh sources that many ponders (myself included) use in their filtration systems. They are beneficial for filtration, but have little influence on kh or ph pond wide. They do their work in the filters.
Your kh is a little bit on the low side at 89, but more than adequate if you do proper water changes regularly.
Now. It is your turn to tell us what you have so we can help you achieve your goals reasonably.
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Larry Iles
Oklahoma
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