In the days before the liquids in carry-on ban, I would bring tropical fish in plastic bags as carry-on luggage. It is still possible to do this but it is so complicated that it is not worthwhile. It involves getting the fish through security out of the water (wrapped in damp paper towels) then dechlorinating and heating some tap water after security, and re-bagging the fish with the newly prepared water.

To transport fish as checked luggage you need a styrofoam insulated box, any pet store will have these on hand as they receive their fish this way. Any pet store could bag the fish with oxygen for you assuming the flight does not leave before the store's opening hours. If you need to bag the fish yourself, fill the bag 1/3 with water then 2/3 with either oxygen (preferred) or air.

You can inquire about the rules for transporting live fish (they will probably demand you pay extra and ship it as freight) or you can pack up an insulated box with the fish in bags inside, then put the whole box inside a duffel bag and the airline won't know or care what is inside the bag. You can add a heat pack to the insulated box to prevent the temperature from dropping too much.

Transporting fish this way is such a hassle that unless the fish are very expensive, you are better off giving the fish away and buying new fish when you get there.