OK i,ve grown quite a few crops but i certainly don't consider myself an expert. Normally i use canna nutrients start to finish, but when vegging i always used to get what i would describe as a mild case mg def with slight rusting on a few leaves. At the min i'm growing a mother in clay pebbles mixed with rockwool(handwatered). Its one of the seeds that i'm flowering off at the min, When i was vegging the rest of the seeds fed on canna aqua A/B i got the rusting, But the mother which i have been experimenting with is perfect in every way. Only different thing i've been doing is 1 feed giving it 0.9 ec with canna then next feed giving it a milder formulex feed to avoid salt build up. I have to say it's working perfectly for me.

Anyone else do anything similar??

i normaly but half teaspoon per 5-10gallon in my res of epsom salts to stop the problem occuring canna is known for low mg levels.

if hand watering you shoild only have to do this once or twice

yes mate, i too always used epsoms. But have found that combining my nutes i can provide anything that the canna lacks.