The best advice is do a lot of reading and research, start with the article above, it explains filtration and water movement. Not to get down on you, but I see issue's with the tank, you will have challenges......
The tank needs more rock, rock is the foundation of the saltwater tank. You should have 25-35 lbs of rock in your tank. Rock is the biofilter of a tank, it provides hiding spaces for fish so they feel safe, and provides breeding space for micro critters. I would suggest to get more rock before the fish, the new rock should cycle before the fish are added.
Need at least one powerhead in the tank, 2 is better, Water movement is important, and most important is to agitate the water surface to help oxygenate the water, the surface should not be flat it creates a low oxygen environement. Algae also loves low flow environments, algae will grow in this tank. Remember we are replicating the ocean, ocean has strong water movement.
Get proper test kits, not API. Get salifert testers or hanna.
You basically built a freshwater tank, but this set up doesn't work well with saltwater hobby. Read the article above for help to prepare the tank properly before you add the fish. Good luck.