I'm writing this article to document the result of adding an octopus to an
established fish tank. On April 15, 2009 I caught a batch of six newly
hatched O. briareus from one of my commercial stone crab traps in
Biscayne Bay Miami, Florida. All six
juveniles were placed into a fully-stocked 125 gallon fish tank
(60"Lx18"Wx24"H). Too make a long story short, only one of the
original six made it, and was named Legs. She emerged at the age of one month
to accept a piece of minnow from a bamboo skewer. It was at this time that I
did some investigation and found out, through research, that I had made a big
mistake adding an octopus to a fish tank.
I made an attempt to catch the fish and relocate them but that proved nearly impossible in a 125 gallon aquarium. So at this point all I could do was wait and see what would happen. Well I did not have to wait long. At three months she killed a Watchman Goby (Cryptocentrus Cinctus) when she took over the cave it had dug in the sand. They shared the cave for a few days, but she soon killed but did not eat the Goby. Seeing that she could hunt proficiently I decided to buy her some live food and began giving her live shrimp as her main diet. Up until this point she mostly ate either silver side minnows or shrimp both bought frozen, then hawed and offered on a stick.
With Legs now on a steady diet of live shrimp, she seemed to become friends with some of the fish, especially a Hawaiian Flame Hawk (Neocirrhitus Armatus). The Flame Hawk would always perch near Legs and would clean up and scraps she dropped. It was September at this point and Legs was five months old. I was very optimistic and things seemed to be going great. I even recorded Legs schooling with the other fish and interacting during their feeding time. However my optimism did last long. Right about the time Legs turned six months old, she ate her first fish a Blue Chromis (Chromis Viridis). It was full grown at about 3". I was shocked because things had seemed to be going so well. It was shortly thereafter that she ate more and more of the Chromis and in very short period she had eaten all seven of them.
Once the Chromis were gone and she realized she had a tank full of food around her she began her killing spree. It was also at this time that different fish started showing their own aggressive behavior towards her. The next loss was my Hawaiian Flame Angelfish (Centropyge Loriculus), The Flame Angel was only about 4" and had become quite aggressive toward Legs. So it was no surprise when the next fish was another angelfish also 4" and also quite aggressive, this one a Caribbean Rock Beauty (Holacanthus Tricolor). It made sense to me that she had killed the angel fish due to their aggression towards her; however, her next meal was the Flame Hawk. I found this very surprising because even up to this point they seemed to be living in harmony taking care of each other. I really felt that the Flame Hawk was going to be the one fish that made it because it showed no aggression towards Legs. The other fish I had faith in was a pair of tank raised yellow stripe Maroon Clown fish (Premnas Biaculeatus). I had never seen these fish interact with Legs and Legs rarely paid any attention to them. Unfortunately, the next fish to be eaten was the smaller of the two clowns, the male. He was about two and half inches.
Up until this point Legs had not killed any fish that had self defense mechanisms like the varieties of Surgeonfish I had. Surgeonfish possess erectable razor sharp spines called 'scalpels' at the base of their bodies just in front of the tail fin. So for the first time Legs killed one of the Surgeonfish, a Blue Tang (Paracanthurus Hepatus) from Fiji. All Surgeons the had become fairly hostile towards Legs and they would swat her with their tail and scalpel whenever she was out and hunting. After she killed the tang I expected the other tangs to be her next target; however, the next fish was an unexpected death due to its size. She killed and ate the tail section of my Florida Checkered Puffer (Sphoeroides Testudineus) which was the largest fish in my tank at over five inches.
Despite all the killing I was surprised that she ate this fish due to its toxic nature but it did not seem to affect her at all. This puffer fish was wild caught in the same area as Legs but a few years earlier. Most of the time the puffer stayed away from Legs and paid little attention to her. He spent most the time buried in the sand. The day after she killed the puffer she killed the large female Yellow Striped Maroon Clown fish (Premnas Biaculeatus).
By December the only two fish left in the tank were a Hawaiian Yellow Tang (Zebrasoma Flavescens) and a Sailfin Tang (Zebrasoma Veliferum) from Fiji. Both of these fish lasted until about the middle of December but they were both killed but not eaten in the end. Part of the reason I feel that she killed the Tangs was food competition. When I would put a fresh batch of shrimp in the tank for Legs, the Tangs were usually able to get one or two of the shrimp. I think this upset her, and motivated her to kill them.
In conclusion, I think it was Richard Ross (Thales) that said it best, “it works until it doesn't”. Keeping other fish with your O.Briareus, may start out fine but will likely end with the death of all the fish.
List of Fish That Were Consumed or Killed