I have a 5 gl hex nano tank I am converting over to marine.
It was great as a small puffer tank, built in filtration and lighting work great.
Then the wife wanted to go big, so now I have a 30gl and 35gl tanks in the front room, and another 30gl salt in the bedroom, along with a 10gl puffer tank.
The 5gl hex was only $50 with everything from WalMart. Its great for small areas if you only want to keep a couple fish or so. I would recommend getting the hex gravel filter online from PetCo, they are around $16 which includes shipping.