I second the use of Quikrete Play Sand. I now have this in all seven tanks in my fish room. The first I tried with this sand was five years ago, and I have had no issues. Rinse it well, but remember that the "dirt" is not going to hurt fish so I never fuss too much, but I do five or six rinses of each batch.
I went into detail with the Quikrete manufacturer, and this sand is frankly the best you can get for substrate fish without spending bags of money. You will only need one bag (you said 5 lbs, I wasn't aware they made this small a bag...I find it comes in 20 kg (about 44 lb) bags at Home Depot and Lowe's. And it costs a few dollars ($6-7 in Canada, will probably be less in the US).
You really do not want 3-inch depth, in a 20g tank. I would say no more than 1 inch to 1.5 inches overall depth (meaning, spread out before you aquascape and build it up in the back, etc). I don't even have 3 inches in my large tanks (70g, 90g, 115g).
Root tabs are fertilizer tabs for substrate-rooted plants like swords. The best I have found is Seachem's Flourish Tabs. The API brand I do not recommend as it can be extremely messy if disturbed, and others who have used them tend not to like them anyway. You place one next to the large plants, and replace in 3 months. I only use these for large swords, aponogeton, red tiger lotus. Obviously if you don't have substrate-rooted plants you don't need tabs, as they do not feed floating plants or those attached to wood or rock. Now,having said all this to answer your question...you might not need tabs in a 20g as you are not going to have really large plants, though one sword can be nice.
Byron.
