Most beginner reef tanks fail for the same reason: impatience. Live rock, sand, and water all need weeks to mature into a stable biological system before a single fish goes in.
The cycle comes first
Nitrogen cycling u2014 the process by which beneficial bacteria colonize your tank and start converting waste into less toxic compounds u2014 typically takes four to six weeks. Skipping it is the single most common beginner mistake.
Budget for a quality protein skimmer and a reliable heater before you spend a cent on livestock. The animals are the easy part; the water chemistry is the hobby.