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Help - PH really high in my first planted aquarium


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I'm hoping I can get some advice here. I started my first planted aquarium a few weeks ago, and all the water parameters are alright.... except the PH.

I'm seeing super high PH, above what I can measure (8.4) and I know it's not from the water I'm using, since I use that in all my non-planted tanks.

Could you help me with what would be the source? Here is what I'm using

- ADA Aqua Soil Amazonia Light
- ADA Brighty K and Green Brighty Mineral fertilizer (very low dose, I have algea problems as well)
- Some stones (I do not know what kind, bought from an aquarium store)



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ADA aqua soil I understand to be slightly acidic (disclaimer, I've never used it) so that seems unlikely to be the cause.

Not knowing what the stones are, they're certainly a possibility. If they contain limestone for instance, that could be adding calcium carbonate to the water, and in turn raising pH. 

ADA ferts I'm not as familiar with, but look suspicious as well. I just looked up Brighty K and saw this rather pertinent note on their website:

*BRIGHTY K is strongly alkaline. The pH of aquarium water may rise temporarily immediately after this product is applied, but it will eventually return to the original level.

I have know idea what "eventually" means in this context, but there's clearly going to be some sort of impact (likely from the potassium carbonate, but possibly from some of the 'inactive' ingredients as well). 

To eliminate some of the guesswork, my suggestion would be to get a few small containers and add 1 of the questionable products to each along with tap water (of whatever water source you're using), plus 1 container with water only. Test the pH of each container each day for a few days. If you're able to test KH as well, that may help other folks on here give you better advice as well.

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