Indicator fish

An Indicator fish is a fish which the Aquarist can use to see how their tank and fish are behaving at any given moment.

For example
Neon tetras or Cardinal tetras can be used to see if any bullying is going on. Normally these fish disperse throughout the tank and only come together if under threat from something.

They also like good quality water and if they start to die off, then there is something wrong with your tank.