On my 75 I'm using two. Originally it was two Rio 2500s, but one of them had an impeller failure and I replaced it with an Aqueon 3000 I found on clearance. If you are only plumbed for one return, I'd go with as big a pump as you overflow can handle. The Mag Drive line is very reliable, and I've heard good things about Eheim and Quiet Ones as well.