You really need to re-pot it or put it in the ground, I think it would rot if you put the pot in a barrel of water, and yes, it would be a perfect place for mossies to breed..

I would
certainly agree with the suggestion to repot or grow the canna in the ground,
but most all canna cultivars grow very well in a summer aquatic setting. If
growing in water be advised to remove well before the winter storage time.

Regards,

Kent

I grow mine in a 3' x 3' foot ground area in front of the house...and they florish and bloom...though its the leaves that are the attraction. I was too lazy to dig to take out the bulbs, which survived the winter, and i live in NYC. Not looking forward though to get rid of those gigantic leaves before the real cold weather arrives.