I don't even think you need two HOB fuges though it definitely can't hurt anything. And get a HOB skimmer like a AquaC Remora. I'm doing perfectly fine with one medium HOB fuge and a skimmer for 25 gallons. And yeah, you can chuck the HOB filter. These are MUCH more efficient then the HOB filter. you can have one HOB fuge and chuck all the filters. Just make sure that your tank is long enough for two HOB fuges plus a HOB skimmer. The included skimmer on the HOB fuge is okay but not really good enough, IMHO. That's why I also have a AquaC HOB skimmer. If it was a choice between two HOB fuges and one HOB fuge plus one HOB skimmer, I would do the second option. One of each. You can always add a second HOB fuge at a later time too.