Do you mean calcium or media reactors?
For media reactors: No, you don't necessarily need a sump. I use and like the TLF Phosban reactor (you can use any media you want). These will hang on the back of the tank (or sump, like mine). You can also hook them together like I did. With mine the first reactor is Matrix Carbon, the second is GFO
, and last is Purigen. You just need a pump to run it/them.
Calcium reactors are different from media reactors and are over my head. I have always preferred to stick with relatively low tech. I don't like things that can make my pH/KH
shoot way up or down, I don't think the difference is worth the risk for me and my tanks.