If one is treading so far down the path of orthodoxy that one must have separate seating for men and women during your wedding ceremony, and similarly prejudiced dance floors, can one still have live goldfish as a centerpiece at one's (kosher) wedding dinner? No, one cannot: cruelty to animals, or Tza'ar Ba'alei Chayim, is forbidden by Jewish law, according to jewfaq.org.
"In the Torah, humanity is given dominion over animals (Gen. 1:26), which gives us the right to use animals for legitimate needs. Animal flesh can be consumed for food; animal skins can be used for clothing... We are permitted to use animals in this way only when there is a genuine, legitimate need, and we must do so in the manner that causes the animal the least suffering."
Does decoration constitute a "legitimate need" under Jewish law? No. The only needs considered legit: "Animal flesh can be consumed for food; animal skins can be used for clothing. The Torah itself must be written on parchment (animal hides), as must mezuzah scrolls, and tefillin must be made out of leather." -ibid.
The well-being of animals is so important that: "We are permitted to violate Shabbat to a limited extent to rescue an animal in pain or at risk of death." -ibid.
Does this usage of the fish cause it suffering or risk of death? Incessantly. Transportation to the wedding is stressful, and potentially fatal, to the fish. At the wedding, the fish may not well treated by your guests, especially the younger ones. If the fish are -- as suggested by a card on the table -- taken home by a guest, transportation from the wedding is an additional stress. If the fish are not taken, as most were not, and you have made not other provision for their future welfare, they will likely be discarded as wedding detritus. "Risk of death" is in every moment of using a live animal as decoration.
"In the Talmud, the rabbis further dictated that a person may not purchase an animal unless he has made provisions to feed it, and a person must feed his animals before he feeds himself (interpreting Deut. 11:15)." -ibid.
A piece of paper on a table passing this responsibility on to your wedding guests in no way fulfills this requirement.
Rest in peace, Tesla the centerpiece (left). As for the married couple, I hope this ill omen can be overcome. And may your future decisions not be as ill-advised as this one. Or as trayf.
"Judaism has always recognized the link between the way a person treats animals and the way a person treats human beings." -ibid.
ADDENDUM: (The next day). Goodnight to you, as well, Edison (right).

photo: Tom Melone