How to determine which out of three experiment groups is the experiment control group? I need help to solve this problem. I'm so confused.
The control group of fish is that group which stays at the unchanged condition. It is in contrast with experimental group(s) where something is changed and we want to study a difference made by this change.
In our case, the control group is that in tank #1. It is stated explicitly that "in tank#1 he fed the fish his usual fish food".
So the answer is 1.