Hi Davi, Water is coming into the fish tank....it leaves the tank by going through the SLO.

It 'looks' like the water is flowing uphill. The water 'does' flow up the long vertical section, carrying solids with it.

The water is actually seeking it's own level....it has to go up the vertical section. Check out the drawing here....
Flow or the water

The top of the tee 'inside the tank' is 'open'....otherwise, it would be creating a siphon.