Cirrus
Arowana blogger
I have a Dreamfish Merlion Harmony Ultimate XB in my care; initially I put the fish in my 300. The aro is close to 7 inches. My pair of BD/Leos are about 12-13 inches across, while my Geos are about 7 inches. I thought it could be a good fit, and it was for the first 24 hours. But then the aro decided he did not like the stingrays. It may be their gliding up and down the glass that scared it, or perhaps something happened in the night, not quite sure; in any case, the aro's "psychology" changed and it went from being the outgoing positive curious aro it was on arrival to being nervous, skittish and hanging out in the corners of the tank. It was still eating, but not with as much enthusiasm as initially. And so I moved it last night into its own 55 gallon holding tank; I will move it again to a 90 gallon tank in a week or so. I plan on keeping it on its own with maybe a few smaller dither fish. I am confident that once the fish puts on a few inches of growth in a few months and gets a bit closer in size to the rays that it will be happy to go back to the 300. For now, the "fish psychology" is just not right. 
Someone called me up yesterday to ask about housing their coming 6 inch Asian arowana with a CT that is 8 inches. I strongly cautioned against it. First of all, their have been two occasions in the past when IT/ST/CT's have killed smaller aros that I have sold. My rule of thumb is that the IT/ST/CT should be no more than 2/3 the size of the Asian arowana. Furthermore, if the Tigerfish has been the dominant fish for a long time prior to the introduction of the aro, it may behave in a territorial manner and attack the aro, or threaten to attack the aro. While an actual attack can hurt the aro physically, threatening behavior can affect the aro's "fish psychology" (lol) and needs to be considered by the keeper.
My recommendation to someone wanting to add an Asian arowana to a tank holding a Tigerfish close in size (but not larger) than the Asian arowana would be to remove the Tigerfish from the tank for a few months and then feed it sparingly during this time (so its growth slows). Meanwhile, I'd recommend power feeding the aro and get it to put on a few inches of size. Also, allow the aro to feel dominant in the tank by removing any other large or threatening fish (large Pbass, Flagtails, etc.). After a few months when you put the Tigerfish back into the big tank, it will now feel less dominant (due to being introduced to a "new" territory), while the aro will feel dominant and territorial due to it feeling it "owns" the tank. This should be a winning combination for the fish to get along. Put it all down to "fish psychology" lol
Someone called me up yesterday to ask about housing their coming 6 inch Asian arowana with a CT that is 8 inches. I strongly cautioned against it. First of all, their have been two occasions in the past when IT/ST/CT's have killed smaller aros that I have sold. My rule of thumb is that the IT/ST/CT should be no more than 2/3 the size of the Asian arowana. Furthermore, if the Tigerfish has been the dominant fish for a long time prior to the introduction of the aro, it may behave in a territorial manner and attack the aro, or threaten to attack the aro. While an actual attack can hurt the aro physically, threatening behavior can affect the aro's "fish psychology" (lol) and needs to be considered by the keeper.
My recommendation to someone wanting to add an Asian arowana to a tank holding a Tigerfish close in size (but not larger) than the Asian arowana would be to remove the Tigerfish from the tank for a few months and then feed it sparingly during this time (so its growth slows). Meanwhile, I'd recommend power feeding the aro and get it to put on a few inches of size. Also, allow the aro to feel dominant in the tank by removing any other large or threatening fish (large Pbass, Flagtails, etc.). After a few months when you put the Tigerfish back into the big tank, it will now feel less dominant (due to being introduced to a "new" territory), while the aro will feel dominant and territorial due to it feeling it "owns" the tank. This should be a winning combination for the fish to get along. Put it all down to "fish psychology" lol