I take it you used the "INSTANT CYCLE BACTERIA IN BOTTLE" stuff.
Some works ok some is not worth it's weight in water.
Before buying livestock you need to research what it takes to keep them alive.
Your LFS (local fish store) is not the best place to get this information.
Many employees are not trained well and many don't care as long as they can sell something.
is a good place to get this kind of information.
They also have a compatibility chart that tell what will get along with what.
As Chi said a month is really nothing to a marine tank most of us take longer than that getting a tank ready to put water in and again longer than a month to cycle it properly so that there are bacteria to de-toxify the water.
Once the tank is ready, filled, cycled, we only add one fish at a time and wait 2 to 3 weeks before adding another so the biological filter can adjust to the bio-load.
Many fish eat only one type of food and a LOT of it, Mandarins are generally one of these. They usually only do well in large tanks with lots of live rock and LOTS of pods.
I have a 65 gallon tank and in marine tank size it is not considered large.
There are many fish that will not do well unless they have a lot of room.
There are fish that are very territorial, and fight to the death to defend their territory, even against larger fish.
If you do research on the livestock you want and are patient you will have more success and have less livestock die on you.