A bucket of tropical fish is a unique item that players will need to breed a specific type of mob in Minecraft, and players can make it with an iron bucket and tropical fish.
That said, before you can make a bucket of tropical fish, you will first need to make an iron bucket with three iron ingots placed in the order inside a crafting table shown in the image below.
After this, you will then need to fill the said bucket full of water, as you will need it to pick up tropical fish in the game, as they can not survive without water.
When this step is done, you will then need to move on to locating the tropical fish. You can find tropical fish if you haven't guessed spawning in the water, specifically the ocean. So if you want to make a bucket of tropical fish, start by heading over there now.
But before you do, make sure to bring some water breathing potions or a helmet enchanted with respiration, as you will need to dive into the water for long periods of time to grab the fish.
Once you got that, you will be set to gather some tropical fish, and when you do, you can use them for breeding axolotl or starting a fish colony.
As a side note, traveling merchants will also sell buckets of tropical fish for five emeralds, so if you don't want to do the leg work, check them when they spawn.