Failure to calculate significance in the correct test! Two-way mixed ANOVA. Pls help!

Hi all, I need some help with my SPSS testing. I am testing how salinity causes a difference in population growth for a species of jellyfish polyp.

I have 3 groups of jellyfish polyps, which remain independent from each other.
Each group of polyps is put under a different treatment of salinity (IV) in seperate tanks (So tank 1 = 26sal, tank 2 = 30sal, tank3 = 34sal).
I counted the number of polyps (DV) in each tank once per day for 42 days (repeated IV) to see the difference in population growth at different salinities.
I performed a Two-way Mixed ANOVA test on this, however the 'significance' values are not being displayed and I am receiving a warning message: "Box's Test of Equality of Covariance Matrices is not computed because there are fewer than two nonsingular cell covariance matrices".
Can anybody please point me in the direction of how to deal with this? I don't understand why it isn't working as I am fairly sure that I have carried out the tests properly, having followed some online tutorials to make sure I did it correctly.
Thanks in advance for any help you can give me!