Besides any differences in max flow rate and/or inputs, the Zakki works via pump suction and has to installed so
the top is above pond level... 7 inch minimum I think?
The Ultra III is gravity fed and needs to be set so the top is just an 1" or so above pond level. Where your pond
is at ground level it would mean you'd need to dig a type of "pit" to put the Ultra III sieve and pump into.