@JuiceBox52 @Retired Viking @Byron Here is what I talk about in another post about knifefishes eating algae, do they need protein and stuff from live food?
The knifefish in the video in post #1 is not eating algae. It is feeding from a prepared disk/wafer which is green. I cannot be certain of the brand, buit if this is for example a Omega One Veggie Round, the ingredients include algae, kelp, whole fish, whole shrimp. Most any carnivorous or omnivorous fish will readily go after this; my cories and loaches love them, and they are not vegetarian.
I doubt that a knifefish will eat actual algae. It may graze biofilms on which algae grows, but it is more likely looking for the microscopic critters in the biofilm. This explains the diet of a Black Ghost Knifefish:
Naturally a micropredator, eating insect larvae, zooplankton and small fishes. Will sift through the sand substrate when hunting food. In the aquarium, they will appreciate frozen or live bloodworms, small feeder fish, tubifex worms, brine shrimp, ghost shrimp, etc. Can usually be weaned onto prepared foods. Small earthworms are relished and particularly good for getting new fish settled and eating. Nocturnal feeders, they should be fed at night. Once settled, they will often feed during the day, even from the aquarist's hand.