Welcome to RC! I really like your design with the aquascaping. From the pics, it looks like the water is still a little cloudy. If that's because you just got done adding sand and water and rocks, then I am concerned for the fish you have chosen. I see a Huma Huma, and a Maroon clown. Two rather larger fishes for a 20L to begin with, and if it hasn't cycled it's far too early to add them to the tank, as they will become stressed with ammonia and probably be damaged permanently, shortening their life span.
So what you can do now. Keep 10 gallons of salt water mixed at all times, and check the water with an ammonia test kit twice a day. If it spikes up very high at all, I'd change out 10 gallons of water immediately. Leaving 1/2 of the water will help develop the culture of bacteria that you want to grow, but dilute the poisionous ammonia. Feed lightly. And don't add anything else until you are sure your cycle is done.
Then enjoy those fish for now, but either start planning for a bigger tank (>75 gallons) or find someone that wants those fish when they outgrow the 20 gallon.
I love the 20L It's one of my favorite shaped tanks.
Cheers!
Hope this helps.
Aaron