What's the most effective way to lower my aquarium's pH?

The best way to lower the pH of an aquarium is to reduce the hardness, as by doing this, the buffering capacity drops. Once the carbonate hardness is lower, the water chemistry can be altered without constant fluctuations - this is why RO water with minimal levels of buffering is favoured for fish that require acidic water.

Adding acids to hard water often leads to a cycle of lowering and buffering where wide variations of pH can be stressful or even lethal to delicate animals. The very best acid to add to an aquarium is carbonic acid and this will happen as CO2 dissolves into water. This is why the best way to keep a stable pH of 7 or less is to use an automated CO2 system in a tank filled with water that has a low carbonate hardness. If the water is too low in buffering capacity then there's a risk of acidosis, where the pH drops to levels that can cause serious problems for fish. Filter bacteria also require carbonates to function but on the plus side, ammonia is far less toxic in acidic water.