What are the correct Ph and Nitrate levels for a marine aquarium tank?

We purchased a new 48litre tank and added 2ml of filter start and 10ml of fresh start as well as 5 1/4 cups of marine salt.. as this is the statistics our booklet with the tank stated to add. We had a gobi fish, pistol shrimp, clown fish and a few others. The nitrogen levels and ph were optimum when we tested them to begin with.. 2 days later the fish were looking limp and then eventually died accept for one hermit crab. We tested the nitrogen levels and it was at the highest point. What effects the nitrogen levels in marine tanks to rise so quick and are there any method known of how to restore these levels quickly and safely? thanks