Azolla
Azolla is a floating fern plant that is great because it can sit in water trays, helps prevent mosquitos by covering water and absorbs nutrients keeping your carnivorous plants in cleaner water. It will spread and divide, so you do not need much. It does not spread crazy like Duckweed, it is slower and very easy to remove. My suggestion is never use duckweed for anything, the pieces are small and it spreads so fast it's almost impossible to remove 100%.
*I do not know the exact species of azolla, it could even be a mix.