How would you
change the hypothesis if mussels died in
both aquariums?

The new hypothesis could be that there is another alternative factor that is negativey afecting dwarfs survivorship.

The previous hypothesis of high levels of phosphate killing mussels is not necessarily disproved.  But the death of mussels from both the control and the treatment groups might be related to one of the controlled variables that could be actually afecting them.      

Probably different levels of phosphate are also affecting invididuals´ survivor rates. However, the fact that mussels from all the aquariums died suggests that there is something else going on that caused the death of all the individuals.    

The control group is used to identify any other factors influencing the results obtained in the study, apart from the modified variables of the treatment.  Individuals of the control group are selected from the same population as the treatment group. These two groups are identical in all aspects except for the independent variable, which changes in the treatment group but remains constant in the control group.

We know that mussels in this experiment were taken from the same population and grown under the same controlled conditions. Only the treatment group received phosphate, but all mussels died, not only the ones that receive phosphate. This result turns to be useful because it is indicating that there is another variable that is killing mussels. Phosphate might be a cause of death, but there is also something else. So the researchers should pay attention to the controlled variables, such as water quality, temperature, pH, food supply, light exposure, or others.  

These variables might be reflecting the real conditions under which mussels live. The results tell that one of these is related to individual death and could be a consequence of climate change or soil use, among other causes.    

Related link:    

b brown mice / white sand dunes