Mechanical removal by hand is difficult and tedious. There are two chemical treatments permitted for aquatic use to remove cattails, glyphosate (E.g., Roundup) and diquat (E.g., Reward, Killzall--a less expensive generic). Using either of these products according to the label instructions should not adversely affect fish.
Diquat only kills the parts of the plant that it comes in contact with, so the roots will still be alive and come up the following year.
Glypohsate, on the other hand, is a systemic herbicide, and will be take up by the cattails and travel to the roots, thus killing the plant.
Glyphosate should be used just after the seedhead forms because the plant is putting all its energy into storing food in the roots for the following year's growth. If you apply glyphosate at this time, it will travel to the roots and kill the plant. It is recommended that you use a surfactant, sold in garden and yard stores, to help the herbicide stick to the leaves better.
Read more about it here: http://ohioline.osu.edu/a-fact/pdf/0011.pdf