What's suitable will depend on the specifics of the refugium space, like how big it is, how water flows though it and at what velocity.
But if I was setting up a freshwater refugium I'd put a ~10cm/4" layer of sand in it, possibly consisting of 5cm/2" play sand on the bottom and 5cm/2" coarser pool filter sand on top. Then introduce some organisms like planaria, Malaysian burrowing trumpet snails, and blackworms, so the whole thing works as a deep sand bed. http://www.wetwebmedia.com/ca/volume_7/volume_7_1/dsb.html
Then I'd add some plants, preferably floating ones like frogbit, water lettuce, or my first choice would be water hyacinth, with a decent light overhead. Alternatively I'd put something hardy and quick growing like Hygrophila that can be rooted in the sand, or Ceratopteris/watersprite.
Lastly, if the design allows for it I'd add a colony of cherry shrimp.
ok cool thanks for help i dont think i can get away with a 4" sand bed tho i think ill do a shallow 2 and use most floting plants the water flow is all acrossed the top kinda fast the sump is going at about 700gph so idk if i want flowing with that flow the over all size is 14" by 14" by 7" tall
ok cool thanks for help i dont think i can get away with a 4" sand bed tho i think ill do a shallow 2 and use most floting plants the water flow is all acrossed the top kinda fast the sump is going at about 700gph so idk if i want flowing with that flow the over all size is 14" by 14" by 7" tall