Some lava rock supposedly doesn't leech minerals, but I just went through a bad experience of the lava rock I was using leeching a lot of KH into the...
Is that rock chalky? Like crumbles when you touch it, you get chalky hands. Etc. I would put the rock out in the sun until it completely dries, then...
I’m not sure if that is lava rock but I’m almost certain that rock is what is causing your high TDS. Remove it and see if it makes a difference after...
