reducer

For a drain… When transitioning from 1.5” to 1” is it better to use a 1.5” to 1” bushing or should you gradually reduce with 1.5” to 1.25” bushing with a section of 1.25” pipe then a 1.25” to 1” bushing?

My aquarium nitrates increase despite everything I am doing. I have a 20 gallon tank with a 20 gallon sump filled with biomedia, 5 gallon bucket filled with pumice with water going through at 50 Gph, 200ml purigen, carbon, 15 oz. Cobalt total nitrate, fluidized sand bed filter v2 bio600f...