It's true that a pH of 8.4 is out of sync with blackwater conditions. It's also true that an RO unit is a good solution. Nonetheless, you may want to spare yourself the expense by seeing how much mileage you can get out of simply treating the water in advance. You can go to Home Depot and purchase a 65 or 95 gallon, thick-walled rubbish container (the one I have is green) and fit it with a small circulating pump and heater. You will need this equipment anyway to age water for water changes. Fill it with water and dechlorinate. Toss in some mesh bags with peat or nuggets, add some Kent Blackwater Pro and let the water circulate for a week. Take a pH reading. If this method is sufficient to drop the pH to at least 7.5, I personally would go with it. Yes, it's not true "blackwater", but it's close enough (IMO). Also, bear in mind that after a month or so, your tank will tend to become progressively acidic on its own, due to all of the metabolim occurring therein. So, combined with the pretreated water changes, you may establish a system which provides the desired pH, au naturale.