If you want to feed your snake fish, you can do it one of two ways: you could put it in a SEPARATE dish in its home, NOT its water dish, that is very bad for its water. Don't want it drinking fish yuck after the fish is gone.
Your other option is to scoop up a fish into a net (careful, they're floppy) and pinch its tail with feeding tongs. Offer your snake the fish just like up would a rodent. Although I do warn, it takes quite a stomach to do this, unless you kill it humanely first.

As for fish sizes, many stores sell feeder fish that are larger and vary in length. Just like choosing an appropriately sized rodent, find a proper sized fish.

I am not familiar with your type of snake as a pet, but I assume you've done your research and know weather it would be good or bad to feed you snake such. Considering it's name is "false water cobra" I'm going to assume it is safe

Personally, one of my baby hognose snakes was such a picky eater, I tried everything and after over a month and a half, fish was the only thing she took. I used the tong feeding method for a while, until I could scent pinkies with fish Strangely, it was cheaper, feeder fish being about 20 cents a piece. If you do feed your snake fish on a regular basis, I would recommend using a calcium booster of some sort, fish do not have the same amount of calcium as a rodent because of its fewer bones