Actually you can have 2 arowanas in a 90 gal tank, as long its wide enough. If its at least minimum of 18" wide tank..its not a problem. I had 2 arowanas, silver and pearl in a 75 gal that was 4 inches shorter than a 90 gal, but same lenght and width.
Silver was 25" whilst the pearl was about 21" or 22". This is going back several years ago, and I have both of them for 3 years and both were doing fine and as a matter of fact they are still doing fine. I don't have them anymore since I moved I gave them away. The silver is now in a 90 gal tank at a friends house and has grown to about 28"...growth may have been stunted a bit, but fish is happy and healthy..no spinal curvature, bulging eyes or curled gills. Same for the pearl (australian) arowana, housed in a 75 gal tank, my old tank at my bro's house and has peaked at 24". May not be as big but both arowanas are still alive and healthy.
As long as you take the time and effort to maintain the tank...you should be okay. BTW, purchased both of them when they were about 2" big.
The pearl was the meaner of the two..used to chase the silver around, but as they grew bigger together they got along.
Sure, the bigger the tank the better..no doubt but if you can't afford it or have the space for it, you can get by with that 90 gal. If you are worried then just get one silver, personally I prefer the austrailian, its the closet to the golden and super reds.
As for me, I'm in the process of re-setting up a new tank, most likely a 125 gal that will house 2 -3 arowanas, possibly a teacup stingray too.
If you do decide to do it, make sure you got a really good filtration system. My old 75g had a wet/dry system that I made out of a spare 20g tank and kept my tank pristine.
Only reason I got into was when I visited Singapore, and a friend of a friend from there took us to the Dragon Fish Industries arowana farm. I swear if it wasn't illegal to have a Super Red arowana, I'd get 6 of them. Heh.
Anyways, while there we asked a lot of questions and these guys are the pros, they breed these babies and know them like the back of their own hand. They told us its pefectly alright to use a 75 gal or bigger tank to house an arowana, as long as its wide enough (min. 18" best 24"). They even stated that if you have a longer tank, 8ft in lengh won't make much of a difference, since they only get real big in the wild or in their farms, since they have huge ponds set up. But indoor captivity with the traditional tanks..not very likely, unless you have a huge custom made tanks like Aquariums do..sure it'll get big, but how many people have that?
If you are really into arowanas, I suggest taking a trip to Singapore. There are other arowana farms out there, and the super reds..omg...what's amazing the price of them out there...hmmm..if I can get my wife to go along with the idea of moving to singapore....lol
There are some web sites that have great info about arowanas, do a google search and you will find some.
Good luck.
They used to keep carp in boxes no bigger than the length of the fish too. Yes they survived. Is it humane or responsible? NO!!! I'm sure you could keep a large fish alive in a small tank. The question is: is it right?
what a dick head