Ph Problems

Hello, So I have been fishless cycling my tank with ammonia and an old filter. I bought national geographic black sand (because it would be vibrant in a planted tank) I missed the fine print saying it could alter the ph... well my ph has been 8.4 or more for some time, akalinity over 300. Tap water is 7 ph, akalinity 150. I put the sand in a bucket on its own and still it was 8.
So anyway I thought I would remove the sand as ph reducers obviously didn't do anything. Yesterday I did a partial water change and switched to flourite substrate, this morning it is still at 8.4 ph, and 300. The only other things in the tank are plants that are now dying and a year old decoration in the tank that didn't have a problem in my old 10gallon the decoration be getting old and causing the issue? My cycle is almost done and i don't know what to do about the ph anymore. I didn't put in moss or driftwood yet because ph was so high I didn't think it would much of a difference...Please Help!

What is the end goal of the tank, and why would you define that pH as a problem? I'm not saying it isn't, as for my choice of fish it would be a major pain in the butt. But what do you intend to put in there? A lot of fish would love those conditions.

I was hoping for a small bodied fish stocked community tank neons, rasboras, shrimp etc :/

Hmm. What happens if you pour tap water into a container and wait a few hours? What does it test at?
My local water changes within a few hours, and there are seasonal changes in water hardness here. It would seem you have an elevated mineral content, if it buffers that strongly.

If I let is sit it stays nice at 7

A bare tank may be the solution until you find out the cause of the pH imbalance : perhaps the black sand, the decoration of the fluorite substrate. If you take out everything except the plants and use the same filter the cycling should still be good (assuming you have not cleaned the walls of the tank).

If you want to start to add fish to the bare setup then add slowly perhaps starting with livebearers (they don't mind the hardness or pH) . In the meantime test the decoration, sand and fluorite separately in buckets to see what they do to the water.