Sorry about your arowana lost its tail Lizzy. Don't worry, if you fish have a big tank with good water condition, its tail will certainly grow back.
One thing unpredictable is that the shape of the new tail will change which may sometimes affect the arowana 's swimiming. If this happen, the arowana may need to go to see a vet to perform "tail trimming", i.e. the arowana wiil undergo anaethesia and a surgical technique will be performed :cut the tail to its normal fan shape; local antibiotics will apply and the tail will grow back in about 3-6 months to its normal shape. Another similar operation will perform for common problem in arowana like 'gill curling".
ETET
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