Back in the day before I started using controlled release fertilizer pellets in water lilly and lotus pots i used to use Miracle Grow tree and shrub spikes (15-5-10) or Jobes general purpose spikes for all trees, shrubs, & evergreens (12-12-12). I would use a set of limb loppers to cut the spikes into 1 to 2 inch chunks and put one chunk in the pot, depending on the size of the pot. I found this to be a LOT cheaper than buying the puny little flower pot spikes. As always, it is extremely important to encase any fertilizer in a submerged pot in clay. Clay provides a molecular bond with the fertilizer nutrients, keeping them from the water column and helping to prevent green water. The nutrients are still readily available to the roots of the plant as they enter the clay. If your area does not have clay soil that is readily available, you can use Montmorillonite, or bentonite. Often this is easily available by purchasing plain kitty litter: read the label-avoid products containing perfumes. Calcium bentonite is preferable to sodium bentonite. Simply mix a little water with the litter to form an elastic mix of gray clay. Line the pot bottom with an inch thick layer, add whatever fertilizer you wish, then add another layer of the clay. Pot your plant as per usual, add a top layer of large gravel or stones to keep your fish from rooting around in the pot, submerge the pot in a bucket of water to soak, then put into place in your pond.
Note: At the time I was potting up anywhere from three to four hundred lilies and lotus every spring, so cost was a big factor.
