Potassium phosphate contains both potassium and phosphate. However, people tend to try to keep phosphates lower to limit algae (though it has been proven it is an imbalance between it and nitrates that ultimately causes algae), so dosing just potassium phosphate may not provide enough potassium to plants.
This why other chemicals such as potassium nitrate and potassium sulfate are often used.
Terrestrial fertilizers are not often used because in order to provide nitrogen, they use urea, which will break down to ammonia.
Dosing then becomes a fine balance between providing enough fertilizer for the plants, but too much that it causes ammonia poisoning.