More higher resolution images are here: http://imgur.com/gallery/sD2T1qN
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Look like the virus pictures I see. Streaks parallel to the veins. The other leaves looking good means it is not due to other growing conditions.
If url not working, search "virus on canna indica" and "researchgate"
It might be good to know which virus in case it can infect other types of plant.
According to the Royal Horticultural Society;
'Canna yellow mottle virus (CaYMV) & Canna yellow streak virus (CaYSV) both causes streaks of pale tissue between the leaf veins. These later die, resulting in streaks of dead brown tissue
Bean yellow mosaic virus (BYMV) causes a yellow mottling of the leaves'
I do not see either of the symptoms in your plants.
Canna prefer warm growing conditions, they are currently having to live through a cool wet season. (You did not mention the night time temperatures, but I assume they are alittle cooler) They are not able to flourish under these conditions. These conditions can lead to rot. You are having symptoms of over-watering. The plant is in a semi-dormant state or a state of slow growth. During these periods they use less water than they do when the weather is warm outside. I would remove any leaves that are very damaged. If the plant's conditions continue into the warm season, you probably have a drainage problem. It may be time to dig them up. Then amend the soil with better drainage matter as deep as you can easily dig. Then replant your Canna. That is only if it continues when the weather either gets much warmer or the rain stops. If the weather does not improve they may prefer to be completely dormant during this time of the year. Some people lift their canna during the cool rainy seasons. Removing the stalks, air drying them a few days then, storing them in a dry medium in a cool, dry, dark location until the warm season returns.