No, I don't think you can run one without a sump. You might be able to purchase one that hangs on the back and has it's own "sump" built-in maybe?
What kind of chemical filter are you using? I use and suggest Chemi-Pure elite it's an all in one chemical filter. It contains ferric oxide to help remove phosphates and silicates. Or you can throw some phosguard in it, too.
I don't use RO/DI water so I take a few extra steps, my saltwater is in a brute 44g in the garage heated to 75 degrees with the stock Fluval 13.5g return pump in it. I added some tap water conditioner also have some Chemi-Pure and Phosguard in there next to the pump. I've been testing for phosphates and there is none detectable in my tank or the premix and I know for a fact we have them in our tap water, because I tested that too.