jasert,
The mesh pots are junk. Best to use pots woth no holes at all. If all you have are the standard nursery plastic pots with holes in the bottom, use a plastic grocery bag in the bottom to seal the holes. Lay the bag in the bottom and add clay up till the holes are covered, then add your fertilizer (controlled release allows for one time per year fertilizing). add another inch of clay, then pot your plant. Add some large gravel to the top and submerge in a bucket before putting in the pond. This method keeps silt/dirt/mud/clay from leaching out of your pot. Additionally, the use of clay (Again, sorry, for those tired of my preaching) seals in the fertilizer, which forms a molecular level bond with the clay, keeping the nutrients out of the water column where it would lead to an algae bloom.