How to buy fresh fish from supermarkets

Hi
in a lot of supermarkets in Japan, i have seen a section where fresh fish is places in water buckets or ice buckets. But i am not sure how to buy them. Should i just pick up an entire fish - place it in a plastic bag and then take to the counter for billing? OR should i take the fish and give it someone who would cut it into pieces and then I take it to billing counter.

Please don't confuse with fish which is prepacked in plastic and kept in fridges. I am talking about fresh fish.

One of the supermarkets that I am referring to is called Sunny Mall which is in Nishikasai area, Edogawa.

Look forward to hearing from you.

Depend on the supermarket.
Often there is a staff to give what you ask.

You could ask if you can take the fish or not.
Also you can check what locals are doing.

You can.
http://www.maruei.info/contact/index.html

Seems that supermarket name is "Maruei Nishikasai-ten(=branch)".
http://www.maruei.info/store/7.html

If no price tag, they can not calculate at cash register.
Did you not ask the staff?

Even in Nishi-Kasai a lot of foreigners who can speak English, but you are non-Japanese speaker?
https://youtu.be/m2o6NBIusrg
https://youtu.be/YPxVel4Jvy8

Typically, you would be expected to pick up an entire fish (or fishes) USING THE TONG placed nearby, place it in a plastic bag and then take to the counter for billing.

If you wish to have fish cut into pieces, you can specify how you want it to be cut as you ask a staff before billing. This has nothing to do with the form the fish is sold in.

I remember you may ask the staff to weigh the fish you put on the bag and they will give you a sticker with the price on it before you may take it to the counter for billing.