@Pond skipper I used to use this recipe I got from someone long ago before I knew about kitty litter. It produced lots of blooms and I also added fertilizer to it so they grew like gangbusters but when the fish got to digging around in it it made an awful mess. If kitty litter gets thrown around it doesn't make such a mess. You can lay some flat rocks on top of the soil so the fish are less likely to mess w/it.
Here is her recipe that I used:
WATER LILY FERTILIZER RECIPE
Use pots w/o holes (or you can line pots w/ black plastic garbage bags) and throw in about a half cup of Osmocote Pellets (Veggie Formula) per 18" wide x 6" deep pot on the bottom of the pot before adding the soil. Osmocote will last for about 4 months. She used composted cow manure for her soil. She said it was the next best thing to natural pond muck. She also stuck a walnut sized wad of plain steel wool on the bottom of the pot. She said it kept the lily from getting chloritic (yellowing from lack of iron). She also mixed in Mosquito Quick Kill Granules into the compost to inoculate the soil w/BT to kill caterpillars that try to eat the pads. When mixed into the soil it seems to "time release" and last all Summer. She mulched the top of her containers w/ pea gravel and if in w/ koi she used bigger rocks or broken flagstone fitted together like a puzzle to keep them from digging around. She said keep the rocks away from the crown so as to allow the plant to grow.
P.S one word of caution: Compost should smell like high quality compost and not cow poop! It should not be moldy,stinky or nasty in any way.
I tried her recipe for several years and I got wayyyyyyyyyyyy too many blooming lilies that covered my whole pond! So if you have the room give it a try.
I had mine planted about 18" deep and in full sun all day long.
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Then I switched to kitty litter, way easier to deal w/and as long as you supply enough fertilizer all is good! So yes you can use soil but keep the fish out of it or one big mess!