As stated above sumps allow you to hide equipment.
In regards to advantages in freshwater allow you to also:
-Increase water volume diluting toxins
-if have species that require low flow have heavy amounts of filtration
-keep plants if keeping species of fish that would destroy plants (goldfish, cichlids, etc)
In freshwater I don't see the benefits as I do in having them in a salt water tanks:
-there's no corals, so controling ph swings during pm is unnecessary.
-regular filters are not nearly as large as protein skimmers or as unsightly that you need to hide equipment as much.
-I've personally never seen a refugium setup in freshwater aquariums that function as well as algae inhibitors as they do in salt water. There maybe examples I've never seen.
-freshwater clean up crews are not nearly as functional so having a place to keep misbehaving ones isn't needed.
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