K.O.I. does not endorse products, but we do sometimes see good ideas that we want to pass along...

One company has developed biodegradable burial urns for pets - including one specifically for pet fish.  The urns or pods are a strong, sturdy construction made out sustainable bamboo and rice husks.  Children can paint or decorate them, and there may be therapeutic benefits, as it is thought that creativity helps the healing process. Perennial flower seeds comes with each pod which can be used to mark the location of the burial.  The pods degrade into the environment in 3-5 years.  The goldfish size is 4.7" x 2.9" x 2" and they make much larger versions up to 22" x 13" x 8" which may be suitable for Koi.   You can learn more about them on the web site:

http://pawpods.com/fish-pod-gold/