Ascites is poultry
Sorry for your loss. You might look at some of the other links about ascites by entering the word in the search box at the top of this page. Casportpony has done some posts with pictures about the procedure. Most hens with ascites are in rough shape any way, so you may have done nothing wrong. I believe that most people let the fluid drip out after the needle puncture instead of pulling it out with a syringe, but I may be mistaken. Was the fluid that came out yellow or amber colored?