Arowana can tolerate temps in the 90's, even your oscar can hang with that. Heat and salt works good, melafix doesn't hurt either. Dosing is 5 ml per 10 gal. as directed. Keep in mind also that warmer water exchanges less oxygen, so airstones could help. Most fish respond well to airstones. It's not so much the bubbles that promotes oxygen exchange, but the agitation of the water surface does. None the less, a little more stimulation wont hurt. Good luck.
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