I do not see an ATS and skimmer performing the same tasks in which one replaces the other. There are many reefers that don't run skimmers, you could do that. If I didn't run my skimmer, I think an increased amount of organic matter would break down and raise my nutrients. If my nutrients were high, I would be looking for ways to export nutrients, one of them being an ATS ( I currently run a skimmer and a cheato refugium on multiple systems, this combination works for me). I also get a lot of aeration from my skimmer that I don't think I would get with just an ATS.