Hmm, one Fluval 3+ is probably inadequate for 120 litre. I have a 50
gallon tank with 2 of them.

How much water movement is appropriate really depends on the species.
Gouramis live in still water in the wild. They don't like moving
water. Moving water also destroys their bubble nests.

My advice is to turn your filter to maximum water flow and aim the
water stream at the nearest wall. This way you have maximum water flow
without the excessive water movement.

You are wrong. The filter does not remove nitrate. It converts
ammonium and nitrite into nitrate.