The Black Shark likes a large cave. You can make it stacking flat rocks evenly and placing a slate ledge across them. I build more above that without silicone, but if a large black shark is startled the whole thing can come crashing down, injuring it or other fish. So try to balance it using the side and back glass for leverage. If there isn't enough room under the ledge you have constructed she will start to dig, so make it large enough she can turn around under there. I have the largest black shark I've ever seen in a community tank with sharks and cats, all at least half her size. Cardinal tetra sized schools become vacation food, but are fine when fed regularly in a well built up tank. I have a rock ledge tear built 4 layers high to the top of my 380 liter. She has the master bedroom at the base. The apartments above caused by the brown slate shelves create detached dwellings for smaller bossy fish like red tail black sharks or semi-large more private fish that need their space. I'd say the red tail black thinks he's the dominant one in the tank, but when the black shark is out, just her size commands respect. If she is startled, she creates violent hydro tornados with her huge tail, sending other smaller fish in the vicinity to go for an involuntary circus ride that can also uproot new plants. I feed her live food now and then as a treat, and she turns from black to red or purple with black scale fringes. I like her darker in colour, but she likes herself more colourful. I've had her for over 10 years without ever seeing another one. I found her in a tank alone in a basement asian aquarium store and she has been very hardy ever since doubling her original size, now around 30 cm. I've never actualy seen her eat another fish, and suspect others for small school night sniping, like the striped Pim I have with his armoured head. I recommend the black shark for moderate non-breeding community tanks. Good friends are red tail black shark, ruby sharks, plecos, chinese algae eaters, and barbs. Must have at least a 200 liter tank for this type of fish to be happy. Plant the opposite side of the tank from the shark if desired. When stressed, they can turn greyish or brown.
I have a 38 cm Black Shark that is kept with a other large fish (mostly American Cichlids) in a 1100 liter tank, and no one really bothers the Black Shark. The only problem is that the other fish get all the food before him. The Walking Catfish gets all the food that hits the bottom of the tank first. He seems to be at ease in the tank though. I don't know if he eats Algae or not because I've never seen him scrape the sides of the tank for it. The Black Shark can be aggressive but they are all bluff because they lack the teeth to do any damage.
We bought our Freshwater Black Shark ("George") in 1981 or 1982. We still have him. He is 45-50 cm long and cannot have any company in his tank, but likes to scare the cats by splashing them. Also gets my attention if I have forgotten to feed him that morning.
My 14 cm black shark is semi-aggressive, but I can't deny that he sometimes converts into a killer type. I can't forget that he killed my 20 cm white oscar and two of my rainbow sharks and some of my baby cichlids (convict and yellow malawi). You have to take care when you are keeping fish like this, you'll love it but it can kill your other fish.
Had a black shark for 7 years before it passed on. Started at 7 cm and grew to 35 cm in a 100 L aquarium! My guess is it weighed approximately 700 grams. I have had several of these in the past 20 years and they are aggresive, but personable. I have not seen any fish that successfully will intimidate them for any length of time. The "big" one was kept alone except for a 10 cm turtle which it used as a soccer ball, i.e., the shark would bounce the turtle around the tank just because...I just got another little one (3 cm long) and can't wait to see its antics!
Our Black Shark just died. It was about 25 cm long and 16 years old. Our shark was very docile and never bothered the other fish in our 280 liter tank.