Hey Jen, I am known to be a little slow at times and three things have occurred to me that may be not helping the issue.
1. I can't remember what the shrimp house was made of, with the subwassertang in the front, was it stone, wood or ceramic?
If stone, it could be a source of buffering up the ph.
2. I noticed the intakes and rain bar of your filter but didn't check it out. What kind of filter and flow rate is it?
3. It surprises me that with your tap water at 7ph that the substrate, with such a good level, isn't buffering it down more. On the fluval packs it suggests to wash the substrate in tap water before using, did you do this? I believe it can reduce the ability of the soil to buffer if rinsed prior to going in.
I am thinking it could be assisted by adding some Fulvic Grain maybe, depending on others feedback on this.
I am sorry not to have asked you these things and paid more attention on my visit, but I was distracted by how well planted the tank is and how healthy all the cherries are going. Cheers