I have a 60-gallon aquarium tank and I have always wondered about the flow of water in the tank. Let's represent the flow of water in terms of a direction field representing velocity, so if we see the tank from one side (mainly the wide side) we will see a direction field like in the image below:
But I know there are areas in the tank that have loops where the water is circular, or where the water is simply stagnant and there is no flow (bacteria can form there).
How would I be able to see the flow of water in my tank? What equipment do I need? Better yet, is there some model I can use to determine the flow in my tank?