Hi all.
I will be setting up a new heavily planted aquarium shortly. I will be using Tropica Aquasoil as my substrate, and RO water (as my tap water is like liquid rock with high PH and Nitrate levels).

Apparently the Aquasoil reduces the PH of the water. As RO water has no buffering capacity, I would imagine that the PH would drop very low (maybe 5ish even). I assume that I should add minerals to the RO water to buffer against this so that the PH will not drop that low. Does anyone know what I should use to do this?

I have also posted this Title on to the Substrate forum as I am not sure which would be best for this problem.

You should always remineralize RO water, regardless of the substrate. I had a similar post on this topic and I also use Tropica soil:

https://www.plantedtank.net/forums/1...help-rodi.html

The recommendation there was to remineralize, but that there is no need to add minerals for kH (i.e., kH=0). I ended up using Potassium Bicarbonate to bring kH=1, which brought the pH a little higher and stopped some of the fluctuations I was seeing. I'm using RODI, not just RO, so my TDS is 0 to begin with. I dose to bring Ca to 25ppm, Mg to 10ppm, and kH=1. This results in a pH of about 6.2. Before adding kH=1, my pH was slightly under 6.

Thanks for the response.

I always use Seachem Equilibrium for the general addition of minerals. As it appears that I should not use KH buffers etc, I assume that should be fine on its own, although I have seen other tanks just using this go down to a PH of 5.5 with CO2 used. I assume that is ok then.
I intend to have Cardinal Tetras and Amano Shrimps and Cherry Shrimps and Harlequins and hope they will ok at that low PH.

Since you plan to use a buffering substrate, you do not need to add anything to impact carbonate hardness - kH - because the substrate will remove it from the water.

Rather than hope a particular animal will be okay in certain water parameters, it's a good idea for you to put in the effort to make sure they'll be okay. Example: Neocaridina (Cherry Shrimp) typically do best in water with higher kH than you'll be able to provide. They may live and be okay but those aren't going to be ideal parameters. Not sure about Tetra parameters but I wouldn't keep them with shrimp unless you're comfortable having them eat or nip at your shrimp.

Unless you're using a pH controller to monitor CO2 concentrations, it's generally best not to put too much into pH measurements when injecting CO2. It doesn't impact carbonate hardness or osmotic pressure, which are what will usually impact your critters.