what are Iranian people doing with the old goldfish at newyear?

iranian new year (apring equinox) celebration is quite elaborate and it spans about 3 weeks from chaharshanbe-soori (celebration of the last wed's of the year which is done tue eve) to the actual tahvil'e saal (the actual equinox moment around mar 20 and closing with sizdeh-bedar (the picnic on the 13th of the new year). the special norooz table that is set up has specific elements and one of which is goldfish. note the all the ceremonies pre-dates islam's presence in iran (roughly about 2,500 yrs) so it has nothing to do with islam. to understand the timing of the persian new year you should also read up on persian calendar which is quite fascinating and by many accounts more accurate than the westerna calendar. it is rooted in the deep knowledge of the ancient persian scientists of astronomy. there are many good sources on-line that you can read. wikipedia and an article from national geographic as well as a few others that i referenced below are just some samples.