Most knifefish tend to be rather timid, and passive. However due to their larger, full grown size, they may eat smaller fish. I know my Ghost Knives cleaned up my 13 Neons...
These fish are easily bullied, and with my experience with red tailed sharks, they may chase off, maybe even bite the knives to get them out of a hiding place.
Also, these knives, while not as big as other, still get about 9", not the best suited for a 40 gallon.
As with all knifefish, it is very important to that they have hiding places, don't expect them to come out during the day often.
One last think, Knifefish are picky carnivores, requiring a high protein diet, start off with frozen blood worms, and move onto krill when they get bigger. Skip brineshrimp all together
