BTW - In Japan, typically a Koi is placed out in mud ponds in the summer, when ambient temps allow it - champion fish do not typically overwinter in ponds. Of course with natural feed stuffs, mud to dig in & work their muscles on, natural UV rays from the sun, and 10's of thousands of gallons of water per fish, generally they no doubt do show a spurt in growth, and an improvement in shine etc. Many koi enthusiasts now believe that with careful management the same results can be achieved indoors, and view the mud pond theory as an urban legend.
Either way, in the application of an aquarium, I don't see any real benefit of adding clay to ones tank.