No need to worry about ph just yet or you can always put in a bag of crushed coral in your filter which you will need to do sooner or later for Africans. Like Onion said, wait for ammonia and nitrite to turn 0 and there's no need to change water in a fishless cycle. Get a squeeze from an established filter sponge (If you don't have any, go to your friendly LFS. They should give you some.) and pour that onto your gravel or inside your filter medium. Add a heater to get the water temps up there like 90 degrees. The beneficial bacteria will grow faster with the heat. Once your ammonia and nitrite are 0, you should see a high level of nitrate. At this point, do a huge water change, as much as possible maybe even 90% and refill the tank with proper ph water for the Mbunas. The bag of coral should help stabilize your ph but test the water before you put fish in case of ph crashes.