Hello and welcome,
You are mentioning some soil with peat in your tank. What kind of soil is it? From what you have described, my first guess would be, that you have something very alcaline in the tank and there is enough of it to keep the pH high even thought you have used so much peat. Something has to have a huge acid neutralisation capacity to have such a tremendous buffering effect. In such case adding anorganic acids like you do will require a huge amount of acids and the changes will not be stable for a long time still which means the pH will go up and down couple times As the buffering capacity will be lowered, the volume of the acid necessary for the change will get lower too. Such manipulation is dangerous for the fish, it is very easy to add to much, as the changes are not linear. As soon as the buffering capacity will get consumed, you will experience a very quick dropdown of the pH, possibly to extreme levels. I would strongly recommend starting over and setting up the tank again using only pure natural stuff like leaves and pure peat if needed.