I have internal filters (Aquatopia brand) in all my boys tanks that are similar looking to that one.I have the flow set to minimum and to make it even softer I place an aquarium sponge (that is rectangle shaped with a hole cut into & down the middle) pushed over the spray bar so that only 1 or 2 holes aren't covered,the water bubbles along out there while the rest just soaks through the sponge.It works great and makes the flow much softer for them.It's very good too because I can move the sponge along as much or as little as my boys like.I have only 1 or 2 holes on the spray bar uncovered for my Veiltail boys & about 4 uncovered for my Plakat boy as he loves playing in the bubbles.
My tanks are 30L & 50L.