I just bought a used 75 gallon tank and I am trying to figure out what type of filtration to put on it. I have an AquaClear 70 from my previous setup and was thinking about getting another one and running two on my tank. That would give me 600gph. According to HAGEN, that should give me filtering capacity to 140 gallons (70 gals each X 2). The Fluval 404 only has 340 gph, but it is rated for tanks up to 100 gals...

According to what I've read in this forum, I should have a min of 750 gph... Any advice or suggestions would be greatly appreciated. Thanks.

The recommendations on the box generally just apply to tropical fish, which have much different needs than goldfish. I would recommend 10x filtration, so 750g/hr as you said. I find my tank is so much easier to maintain with more filtration