cat litter- MAKE SURE IT IS 100% clay, no additives. Don't use clumping type or smell eliminator types! Clay is VERY beneficial to all types of of carp, Koi and Goldfish
osmocote- BAD IDEA! It's a slow release fertilizer than can increase your nitrogen
significantly over time, It also is not in the range of what your lilies need.
Oil tubs- great idea, low sides large bottom. perfect for growing large lilies! ONLY use new un-used ones or ones that have only been used for pond use. ALWAYS rinse any product no matter what it is off before placing in pond! Even new products can have contaiminates, I learned this lesson years ago! May have been air freashner, bug spray, or someting else but I placed a planter in a small water garden tub that had been sitting outside on my work table. Forgot to rinse it, withing two hours all my fish had died.
pea gravel as a soil substitute. Not good, no way for the lily to get good root growth. Bottom half should be filled with a WATER planting substrate or (KITTY LITTER), If available river sand does VERY WELL. pea gravel on top, great for goldfish, sucks for Koi! Koi love to get it out a swim around with it before spitting it out. Your pot's will be 75% empty by summers end. use large gravel on the top if your koi are over 6" this will stop them from removing the rock and stop most digging.
fertilizer in ponds- I use Highland rims Aquatic Plant fertilizer, On tablet per lily per month during the entire growing season. I have 7 lilies in the pond. All bloom and last year no green water although I did start using a UV light at the rate of 1 watt per 100 gallons. You do not want to use ANY fertilizer in ANY pond plants that are not designed for ponds Period, They are made to be safe for fish, Non-aquatic fertilizers are not designed to be fish safe, If you can't use aquatic fert. tabs don't use anything except cat litter. the lilies will grow from the nitrates that are produced in the pond, they won't bloom as well, but they will grow and should bloom some.