Sure you can, but I would not recomend doing it as its presently setup. If its freshwater fish, it would be no problem, but best bet for saltwater would require some changes. I wold pitch the undergravel filter, and marbles and start with just the tank. Add about 3/4 to inch max of live sand, and some liverock at the rate of 1.5 to 2 pounds per gal of water........Since the tank is so small, you can get my with a typcial filter called a NANO filter which is a hob type power filter.......or better yet get a aqua clear20, and turn the filter portion of it into a small refugium with some liverock rubble and cheato or calupera (sp?) A simple clamp on light of the power compact with a 13 watt 50/50 bulb is more than sufficient, and once its all cycled, you could throw in some mushrooms and other low light requirement critters. Cooling is usually an issue with the small tanks but its still a very doable project. Do a seach for PICO tanks..........Its being done all the time.......Its more of a challenege overall keeping things right than working with more water volume as small minor things are usually more noticeable to a small tank.........A weekly or every 10 to 14 day water change is usually sufficient, and once setup and you can maintain temps and water parameters, you could easily keep a fish in it.....Neon Goby would be a good choice. They do well in a specimen tank by themselves, and you can always add a snail like a cerith for algae control, and possibly a few tiny hermits........may even luck up and get a hitch hiker serpent brittle star in your liverock.......I have a few pico setups all around the house and to tellyou the truth I actually enjoy them more than my larger setups.......I wold definately recomend a AC20 powerfilter modded with a refugium though as it will add a bit more water volume and help in keeping the tank parameters better.