I have that exact tank I used it for a Betta. It is sitting idle at the moment. But I have considered making it a salt water pico tank.
Things to know about the tank.
1. Those petco filter media suck and replacements are overpriced. Take one of those two cut off the screen, remove the media place it in the back space. Fill that front chamber with a sponge between the plastic media part you placed in and the overflow.
2. The pump that comes with it is under powered. Get an ajustable speed 70gph+ pump. It comes with a 40. 40 will work for a fresh water betta fish, but not a saltwater tank.
3. At some point you may want corals. The light that comes with it is not for corals. You will need to replace it with a PICO reef lighting solution. The light is fine for fresh water, but not reef. It will however work for non-photo synthetic corals, and low light corals like mushrooms probably.
4. in the second chamber you want to place in a home-made caddy. Something like this. but really it just needs space for two pieces of egg crate spaced apart and hanging to the side. so water runs over it. On them you will place a bag of carbon, or a bag of purigen, or phosguard or seachem matrix depending on what you need. I would use chemipure elite for one and seachem matrix for other. Reason for chemipure is it is a two in one solution, basically carbon and purigen type material all in one.
5. The tank is too small for most salt water fish. But if you check the PICO section of forums they should give you ideas on what will work in the tank.
PS.. Super Derek basically has the same advice for the chambers. first sponge, second media.
Anyway good luck. I am sure this tank will make an amazing PICO tank. It is very high quality for it's size. One of my favorite tanks.