The problem with using a fert like Canna is you never really know how often to feed. There's alot of bad info on the web and I'm not sure how all this stuff got started but the feed/flush idea has been around for along time, but it's bad info. You don't need to do anything special to grow this plant but for some reason growers always think that the plant has special needs and if you do certain things, the plant will give the best results. All that needs to happen is just feed once in a great while, or when the plant appears to be deficient in something. Then you just water when the soil is dry. Flushing so frequently does not guarantee that you flushing the ferts out of the soil, if that's what you're trying to do. All it does is keep the soil wet, which really isn't good for them, staying wet all the time. This could be the reason why you're seeing dead roots. There are bette ways to fertilize for people that really don't know what they're doing, which is to use some powder ferts - one half cup of powder (not crystalized but powder) for every three or four liters of potting soil. For some reason, powder ferts don't seem to be popular at Ireland grow shops from what I can find on the web. There are at least two companies in America that sell this kind of fert. On this page Foxfarm has theirs and I would recommend the Fruit and Flower one in the yellow containter. FoxFarm Soil & Fertilizer Company
It wouldn't be cheap, but I bet you could order this stuff and have it shipped to you but just be careful that you only order one box or bag, not more than that. If you don't want to try this, you don't really need to, that's fine, but just be careful in the future of how much Canna you give. It should be a very good fert but you've used to much on that one plant. The powder ferts are pretty complete with nutrition and shouldn't need anything else. Canna's the same. You could go ahead and use a little seaweed but not much because it's strong.