Ok first off who is "they"? By the look of it I would say you got the aqueon 10 gallon premium tank set (or something similar) so I'll assume you bought it at petco or petsmart right? If so then they probably have no idea what they are talking about. Just by looking at their livestock ID tags I know that they just want to sell you what they can. The ID card for an oscar fish says they hit 10 inches and need a 30 gallon tank. I have an 11 inch Oscar and they can hit 16 inches. They also need a 55 gallon tank alone. So when they say to keep that filter in there they are full of crap. The filter cartridges go bad once the carbon has worn out. Once the carbons bad it's just a bio holder. Keep your HOB I was just saying I wouldn't use one with a saltwater tank and for that reason.