Starting a new tank and high pH from the get-go

[Herefishie]

I purchased a Fluval SPEC 2.6 gallon with the intention of housing shrimp only. I have two other tanks. I use tapwater and all the parameters are fine in those tanks. I set this shrimp tank up 3 days ago. It has gravel substrate and a few plants in it. I tossed in a sponge pre-filter from one of my other tanks to help seed it. The pH is high! It keeps reading 8.4. I can't understand why considering I'm using the same water. I replaced half of the water this morning with RO water but it hasn't changed. I'm pretty new to the hobby and sure would appreciate some advice.

[Xaos]

Some types of gravels can increase pH, and in a small tank that’s my guess. Replacing it with another substrate could help, but if your RO water is testing acidic <7.0 you could keep at the water changes and see if that brings it down.

[Lennie]

It might be due to you are reading ph out of the tap directly, without aerating it.

Aerating increases ph. 

Try to aerate your tap water for some time and test it again to see if it will read any higher

Edited March 11, 2023 by Lennie