Here's a thing I pasted from another thread -
A basic marine set up is like this. don't mess with anything less than a 3 foot tank. Smaller can be done but it's hard. 4 foot up is best. The filtration is done by bacteria that live in "liverock". Liverock is rock made from old coral skeleton and is porous. You should have somewhere around a kilogram of liverock to each 8 litres of water in the tank. The liverock is in fact dry and dead when you buy it at the LFS, but becomes "live" when after a few weeks in the tank it gets a bunch of bacteria living in it that eat the waste products from the fish.
So one way to set up a tank is to put a thin layer of sand in the bottom. Put in the correct amount of rock, then fill with seawater. The seawater will need to have 10% freshwater added as our water is saltier then the ideal level.
Then you need all this water to be moving, both to assist liverock filtration, and because most marine organisms need it to be moving. So you add a few pumps, enough to pump the entire volume of the tank ten times per hour (10 x times flow) This is the minimum, 20x or more is better.
Then you need a marine grade light, most of the corals we keep are photosynthetic and need correct light. 1 watt of light per litre of water is a rough guide to the amount of light you need, but this may vary (upwards mostly) depending on what you are keeping.
Don't use your old cannisters for biological filtration, leave that to the liverock. Cannisters and such do aerobic filtration only and do not supply the anearobic filtration important in a marine tank. Cannisters can be useful for using carbon, or phosphate removing resin though.
The other thing you need is a protein skimmer. A good one is expensive, but the success of your tank will to quite a degree hinge on how good your protein skimming is. A good one will last you for life, get a needlewheel one they are the best.
As I said you will hear other contradictory ideas and they will likely also work. I've just suggested one way to get a basic system going.
Do a lot of reading, that's the big thing, there's a lot of learning to do.
Good Luck!