Black and bowns get no more than 8 inches in an aquarium, they only get larger in the wild. you might be confusing black and browns with clowns fish boy.
Knife fish can choose to attack anything they choose to, but fish of a similar size are probably not going to be messed with. Similar sizes and bigger are just not on the menu for knifes feeding habbits. Im not saying a knife attacking a similar size fish is impossible, im saying its improbable. The only agression toward similar sized fish i've seen is when knifes are placed in new tanks and they will run off other fish to , in essence, establish a pecking order.
Knifes are nocternal creatures with an instinct for ambush hunting, so yes, they like the night. however they DO in fact come out redily during daylight hours and can be easily trained to eat during the day, even hand fed life foods, which i have and are doing at the moment with a small clown knife.
Yes, you will see them out in the evening, thats a given, but strong plant lights have no visible negative effects on knifes.
They do best in heavily planted tanks given their natural environments. This also caters to their ambush predator nature.
These fish can be kept with a number of other species, and are fairly gentle creatures. I've kept them with angels, goldfish, labyrinth fish, pleco's, snails, sharks, etc. They will defend themselves if attacked however.
They are predators, so they require life foods. I have trained some to grab at flake, however that should not be a staple food. Mine do great on a diet of frozen brine in chunks and life guppies and minnows on occasion, which i only add in while the tank lights are off simply to cater to the instincts of the fish, however they will hunt while lights are on, they are opportunistic in fact.
I hope i've dispelled a few old wives tales about knifes
