Lava rock is fairly standard issue for ponds. I don't think they would be toxic after aging. <Fresh lava is a different story.>
3 possibilities:
1. Your tap water has dissolved CO2, and the pH naturally goes up after sitting out for a few days. <Test by letting some tap water sit out for a day or 2 & seeing what the pH is.>
2. Something in your tank is dissolving into the water - shells, limestone, etc. The rocks are suspects. <Test after you confirmed that tap water holds stable pH. Soak some of the rocks in tap water for 3-7 days & see if pH rises.>
3. Ammonia is basic, adding enough will alter your pH. <Did you test the pH BEFORE adding ammonia?>