I'm sorry to hear about your pullet.Hello everyone and thank you for taking the time to read this and offer advice.
I have a Cuckoo Marans pullet that I noticed was looking pretty bare on her backside, so I took a closer look. She is filled with fluid. Pretty sure that it's water belly. I did not take any pictures, but if needed I'll get some in the morning.
She hatched last Aug, so isn't very old. She has always been a very heavy lady. Just figured it was because of her breed. She is my only Cuckoo Marans. I am planning on putting her down. I am not equipped to drain her and I don't want to see her suffer. She is a well loved pet. I also would like to know why she got water belly. I thought only broilers and old hens get it? I mix a bag of 20% all flock with a 16% layer feed. I did that because I started having problems with soft shells in my flock. They do have os and toasted egg shells on the side but still had problems. Did I do wrong by mixing the feed?
Also, I'm thinking I would like to try to do a necropsy. Does anyone have any links to step by step instructions on how to do that?
Sorry for the long post but I appreciate the ones who took the time to read this.
I see you have already recieved good input/suggestions so far.
I think you may need to give a little more information.
Is she still laying eggs?
Can she eat/drink on her own?
Does she have a good quality of life right now?
You mention you don't want to see her suffer - so can we assume she is not feeling well? She's acting sick? What are her symptoms?
If she is not sick right now... then you may want to just monitor her. Draining is an option to look into. You can find syringes and needles at tractor supply if that's something you want to try first.
Not all hens/pullets can be saved, nor should they - each of us have to make a determination of the quality of life they have and the time/money or experience we have to offer. Ending suffering is always an option if this is what you feel needs to be done, it's the right thing to do.
You did nothing wrong

We can try to help you if you want to treat her. If you have made the decision to put her down and want to try necrospy, that's good too.
Have someone take photos and post them so we can help you with what you see, we all learn along with you.
I would look in the abdomen first. Working your way on into the organs. Liver and heart dysfunction can cause Ascites as well as reproductive problems.
The following link may be helpful to you. Also the necropsy thread here on BYC may be helpful, you can see what others have done and what they have found.
https://vet.uga.edu/oldvpp/programs/afvet/attachments/how_to_necropsy_a_bird.pdf
https://www.backyardchickens.com/th...ning-contains-graphic-necropsy-photos.823961/