It's interesting that you're seeing roots out the bottom already. If you transplant, you probably should go with large containers, to minimize the need to do it again. 5 gallon buckets or bigger. I'm guessing that coco is a good substrate to use when using the canna brand. Not that you couldn't use potting soil, but if Canna provides everything needed, then coco should be fine. please take photos in white light next time so we can see the leaf color better. I've never used coco so can't say if the root growth and slow plant growth are due to the coco or what, but usually plants grow root more because they are looking for water. If coco doesn't hold water to well, which is the reason why potting soil uses perlite, the maybe more water is necessary. Roots also need some dry cycle to come in contact with oxygen. this is another reason why perlite is used in potting soil, to create small air pockets. I'm not sure that straight coco is best. You might consider using a potting soil at transplant. Roots brand, foxfarm, promix, happy frog seems to be a good one. They all have their own ferts to them, so be careful how much fert you add if you switch to a soil. Also, you probably won't find a schedule to feed. Sometimes the fert makers don't tell you how often to use the product and let the grower figure that out. If it seems like water runs through quickly and the coco dries quickly, it's probably too quick and you should switch to a potting soil. I think pythium is water-based infection, so perhaps if you use distilled water you won't have anymore trouble with it.