Hi,
There are several conditions :
- both must be more or less the same size,
- avoid introducing them if they are less than 4 inches long. Under this size, they have strong cannibalistic trends and tend to nip each other's limbs or tail;
- they should not be stressed before introducing : temperature Ok, not too much nitrates, ...
- they should be very well fed before they meet.
If possible, introduce the new axie in the evening.
Even if all these conditions are met, check their behavior during the next days.