PH help please.

Been googling but can't come up with an answer.
Ammonia ok (negligable) also nitrites same but ph is very high.
How do I lower this to the recommended 7.6?

Also how do you grow plants in the tank, do you use a small pot or such?
I did have some supplied from a shop which didn't seem to grow roots and
appeared to be happy floating on the top.
Three small healthy goldfish in a 16 gallon tank, and I would love to keep
them that way :0)
Rog (UK)