Water soluble is by far the easiest to use. IMO the second easiest way is regular granular product (several types (0-46-0 + some 13-13 13 or even some 45-0-0 can be mixed to give the right mixture) placed on platforms. The most difficult is liquid as you have to mix it and wash it around the pond by motor or sprayer. You can't just dump liquid into the pond.