Need help on freshwater sump

[Twaldo]

I have a 75 gal tank that is drilled and used to be a saltwater tank, anyways I understand how a saltwater sump works but can someone kindly explain to me how a freshwater sump works ? I have look at many and many diagrams online and still I am a bit lost
Thanks

 

[Crispy]

easiest thing to build would be a wet/dry trickle tower. you can use stackable drawers which are cheap and easy to work with. doing a bit of searching should give you an idea of what you can do.

 

[FAT daddy]

If you use filter media like pot scrubbers or bio balls inside the wet/dry trickle tower the large surface area of the pot scrubbers or bio balls allows for more beneficial bacteria to live on. Those bacteria help breaking down ammonia and other sht.

Steri-lite shelving units work well but you have to drill holes through the top to allow water to flow from your display tank to your sump. I used a big sterilite tub for the actual sub but since you kept saltwater you probabaly have a legit wet/dry to begin with.