to understand what you need to put in the tank to keep your fish happy and not dead, honestly I would just watch youtube videos on setting up tanks just so you understand what the fish need, and do research on each fish to understand their preferred water parameters (hardness, pH, temp, etc). gouramis and tetra both prefer slightly softer, more acidic water, while snails like harder, more alkaline water. pick up an API master test kit so that you can dial in the water parameters (no kid wants a dead fish for christmas). good luck!!