I can't afford those so I use fruit tree spikes and break them into chunks small enough to stick down in the gravel. I only fertilize twice a season. Maybe I am under fertilizing?

I use whole fruit spikes (4-5) every few weeks in my lotus bog, but I'm not sure that's even enough. I used to use very small tomato spikes in my 7 gallon water lily containers But I do think I'm under fertilizing the water lilies for a couple years. It's hard to know how much you're giving in those chunks of fruit tree spikes. I would just go by their response. if they seem happy and are making blossoms, I think you're using enough. I had problems last year with my water lilies, and maybe it was because I wasn't fertilizing enough. But like I said.....if yours seem happy and productive, I wouldn't change anything. But...........only twice a season seems sort of low. Would you say your lilies are doing well?

Jobe's tomato "spikes" they are really little tablets. Nitrogen is nitrogen. I don't pay extra for "pond" fertilizer.

While plants need phosphorus, they don't need much. The whole, plants need phosphorus for blooms, is marketing hype.

Phosphorus is phosphorus too. You don't need a fertilizer labeled "for ponds" to be useful in a pond is what I meant. I go by the ingredients and ratios rather than what species it is labeled for. I do use a lower Nitrogen fertilizer on plants I want to bloom or fruit more. Which is what I was taught in horticulture class many years ago...