I use a fluval 305 on my 55g and have never had a problem with it. I cleaned it monthly and never nitrates in mine. It;s just a matter of maintenance is all. I would have to agree though that an aquaclear is an excellent filter for saltwater, you can change the media to what YOU want and need as time goes on.
If an aquaclear I would go with the 30 or the 50.
I have an AC50 on my 20g coral tank now and its working out well.