We use tablets in our lillies and lotus. Similar to this:
http://www.stillwatersnursery.com/pondproducts/pondfertilizer.php
But really, any tablet will do. They are all primarily the same thing.
What we found works well is this:
Get a PVC pipe that the tablet will slide down into. Usually a half-inch schedule 40 PVC works, about 12" long works well.
Get a second PVC pipe that will slide down into the first. 1/2 CPVC pipe works nicely in the 1/2 inch schedule 40. This should be longer, 18" is good.
Push the larger PVC pipe down into the pot about 3-4". Can be tough sometimes.
Drop the tablet down into the tube.
Drop the smaller 1/2 pipe down on top of the pond tab.
Now, as you hold the smaller pipe down against the pond tab, slide the larger pipe up and off the smaller pipe.
Push in the soil/gravel around the smaller pipe.
Pull the smaller pipe out and push in more soil/gravel into the hole.
The pond tab should now be buried about 3" into the pot.
This is great for mid season fertilizing and can be done without pulling the pots out of the pond even. Most of the time as you pull the smaller pipe out, the soil and gravel will just back fill by itself without any help.
This helps ensure the fertlizer stays with the plant! Takes a couple practices but once you do 2 or 3, you'll be able to do pots 2-3 feet out in the pond.
Craig