| The bass and the arowana will outgrow a 90g tank rather quickly, especially with the both of them in there at the same time. They'd likely be okay tankmates, though. All three are Amazonian fish, so you'd have a really neat biotope. For the long-term, I would suggest a tank that's at least 6' long by 3' wide, and as deep as you can get it. This will give the arowana and the bass space to swim around in, as well as extra water to increase the bioload capacity.
The clown knife should have no problems from the arowana, as it sits on the bottom and the arowana hunts the top. I would think the knife would stay big enough to not be eaten by the bass, but I'm not sure.
Edit: By the way, if you plan on guppies being their primary food source, I would suggest starting your own feeder guppy tank. The ones sold by pet stores are really low in nutrition and rife with disease. If you raise your own, on the other hand, you can feed them really well, making them very healthy. Healthy feeders make for healthy predators. Last edited by sirdarksol; August 30th, 2008 at 10:03 PM.
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