high PH yet low KH & GH?

NaOH is strong (defined as completely dissociates in water into Na+ and OH-) but only tiy amounts are added, because it's only a small shift required (I know as fishkeepers we think of a couple of points as large but in terms of what you can do with strong bases it's small) so the peat will have a significant effect.
 
As mentioned prev, peat will have neutralizing effect but depends on better analysis of tap/RO water, not just with an assumptions.
For fish, pure vinegar would work better but again all depend on OP's chemical properties of water.

If pH of RO water is 8.8 @ 0 KH/GH with hobby test kits, I wonder what its true values are before RO???
Thus recommended the analysis of straight tap water by pros. At lease such will reveal if water is safe for human consumption/usage as well as how to approach in appropriate preparation of tank water to suit OP's needs.
 
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Let me play junior chemistry sleuth here.
Hardness is a measure of divalent cations. KH is a measure of carbonate. Every divalent cation (or monovalent cation) has to have a counterion. You are only measuring for one, carbonate. I would measure for phosphate, because di- and tri-basic phosphate can raise pH to the level you have seen. Sulfate and fluoride are very poor bases and would be very unlikely to have this effect. If you have a LFS, call them and ask them if they can measure TDS in your RO water.
If it is phosphate, your ion exchange cartridge may be bad, if you have very hard tap water going in that may be the reason.

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"teh googles" seems to show this happens, here, and here, for example. TDS and bad membranes are potential sources of problems.
 
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