What method shoudl I inthose cell biovolume data

Hello everyone!

I am wiring a masters project involving counting algae and I am having a difficulty finding what is the right method to do the following comparisons on the biovolume data in this picture bellow:

To test if there is a significant difference between the pairs Inlet 1- outlet1 , Inlet2- Outlet 2 , Inlet 1 - inlet 2 and Outlet 1 - Outlet 2

Because I found the data are very skewed and not parametric I calculated the differences of the above pairs and run an 1-sample Wilcoxon test in minitab.

However I found this test to give P way above 0.05 so I wonder if I am using the wrong method for this type of test

Can someone who is more proficient with statistics of this sort help me fight the right method for doing this right?

So far the only way I get a significant result is to get rid of the zeros by adding on all values a small number like 0.0000001. Is that the right way to deal with those data?

Thank you in advance