It is a lightly planted tank with manzanita wood and aqua dirt as a substrate. I have an ayirstone in
Do you mean aqua soil ?
If so,this sometimes depending on type, contain's organic matter that can give rise to ammonia and or buffer the water depending again, on what type of the aqua soil was purchased.
Some info on the specific brand/type would be helpful.
Would still use dechlorinator for the volume of water the tank hold's, at water changes,and mix water for water changes in tub or rubber maid barrel ahead of water change day.
Not sure R/O unit's remove chlorine or ammonia from chloramines so good dechlorinator such as PRIME is good insurance.
Could be ammonia from aqua soil if the soil is new,and or buffer's in the soil giving unexplained ( to me anyway) rise in GH,pH, after 50/50 mix of R/O and tapwater .
Maybe R/O membranes/cartridges need replaced?
Just spit ballin here .
Maybe try 60/40 mix of R/O tap water in bucket and test after 12 hour's/24 hour's.