This are the fittings I bought
This fitting I clued behind a piece of metal so no water can escape and used it to put on top of the fitting connected to the pipe so that water does not come out when the compressor is off.
The fitting and the compressor just fit into it, and there is a release spring to remove it.
The compressor came with all kind of goodies:
Connected the fitting to one of the inlet pipes after I cut it.
Here is the compressor without water flowing
and here it is with water flowing:
This is how my sand
looks like after cleaning
And here is the filter and the compressor, next to my filter.