Question about ph

I am geniunely so confused as to why my ph is above 7 rn. I have a tank that is been filled for a long time with water, and just sitting there because I never got to it. It is fully scaped with aquasoil, but has no plants or livestock. I want to actually finish it, so I changed the water with ro water(not remineralized). I tested the water a couple days later, and it is above 7. All it has is old aquasoil, and completely inert hardscape, like lavarock and wood(if anything this should lower it). Why is the ph so high?