The first thing to realize is that everything is different. You don't need "another" filter. You need a completely different filter. The filtration you have on your freshwater tank will not be used at all on a marine aquarium.
The goal in a marine aquarium is to directly remove organic waste, without allowing the waste the opportunity to be broken down by a biofilter. This is accomplished with the use of a protein skimmer, live rock, and aragonite sand bed. It doesn't matter if you are doing a reef or fish only tank, the setups are identical.
For example, I have a 54 reef and a 180 FOWLR. Both use this same concept. The only thing that is different is the lighting, because reef systems require more intense lighting. In your case you will save a ton of money on light, but the "filtration" will be identical. For a 90 gallon tank, you could get away without using a sumjp. This will save you a ton of money. To do so, you would purchase a hang on protein skimmer. Keep in miind, the protein skimmer is the single most important purchase you will make for yiour tank, so this should be a relatively expensive purchase. For a 90 galloni tank I would suggest this:
AquaC Remora Pro Protein Skimmer with Rio 1400 Pump
You will need to buy the surface skimmer attachment:
AquaC Surface Prefilter Box
This will be all of your filtration. You will not use any biowheels or other biological filter media. Doing so will actually hurt the environment by removing carbonates and increasing nitrates.
For rock and sand, I would suggest ordering a 75 pounds dry rock shipment and 160 pounds of sand from here:
75 Pounds Key Largo Rock, <br>160 Pounds Bahamas Aragonite Sand<BR>pay $35 shipping only - KL75-160
You will then need to add 15 to 20 pounds of live rock from your LFS to seed the dry rock.
If you follow this method of setting up a marine tank, you will be successful and find the hobby to be very rewarding. For more details on this concept, here is a link to another thread: