First post questions about arowana in the arowana sub form. Less chance of getting bad advice, if there is bad advice it gets squashed.
Either the 55g or 29g will be fine. If you can, lower the water line to 7 or 8 inches below top of tank. Add some floating plastic plants for cover. Keep the temp up to 82 degrees and do enough of a waterchange to keep the nitrates around 10ppm and the pH stable. Keep without tankmates, if you feel the need for tankmates post question and get opinions, remember baby aros are very fragile.
Baby aros are very skittish, so you will need to feed food that floats for awhile. Also they need to be fed soft food while this small. Best foods are freeze dried blood worms, a quality flake food, and a soft freeze dried baby shrimp or krill.
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