While waiting for that answer, let me also mention that although your tank isn't overstocked, which is something many newcomers to the hobby do, your tank dynamics should be looked at.  Tetras are schooling fish and need to be in groups of at least six fish.  Danios are the same, and in addition to this requirement they need a great deal of swimming space.  20 gallons is an appropriate size for these highly active fish.

Well, let me edit a bit.  Just wanted to say good job on acquiring live plants - they're attractive and very beneficial to your water chemistry.  However, anubias is a low-needs plant; it gets by with very little light and thus, fewer nutrients than other plants.  CO2 shouldn't be necessary at all with this species - it's not something you need for all planted tanks.  Also, what type of lighting do you have?  How many watts is the bulb(s)? 

If you're planning on purchasing higher-needs plants that require more light and will benefit from CO2, the system will be more worthwhile.  If you're interested in setting up a more heavily planted tank, the section "Plant Particulars" is a great place to ask questions and get more information on how planted aquaria work.

Good luck!

You can make your own comparable CO2 system for a fraction of the cost using household materials, though, if you decide eventually that a commercial brand isn't your thing.

There is an article under the topic "Only 2 Neon Tetra" on these forums that explains in-depth why schooling fish need schools.  Yes, they are hardy fish and look good, and are good fish for new fishkeepers to start off with.  However, they are not at their best with only three of their kind and are experiencing significant stress, which is a major contributing factor in illness and overall decline in health. 

Aquarium stores can have knowledgeable, helpful staff that are wonderful assets in setting up a first tank, but it's always important to do your own research.  Getting second opinions and reading materials about fishkeeping should always be undertaken before setting up a tank or purchasing any fish. 

You are currently running four watts per gallon, which is in the "high light" range.  Anubias species will be happy in much less, however, you'll get lovely growth from the plant under higher light as well.  However, anubias are slow-growing plants; do not expect a rapid amount of growth out of the plant like you would get with certain stem plants.  With the amount of light in your tank right now, you will be able to grow basically any species you desire.  However, algae also enjoys higher lighting, so be on the lookout and be prepared to remove it.  Use a simple scrubbing tool (I use washcloths and a toothbrush) to get rid of algae; you don't need chemicals.  Be careful not to overfeed with fertilizer either - algae loves spare nutrients that your plants don't use up.

There's really no need to wait to plant your tank further; if the conditions are right - they will be ideal once you get the CO2 system working properly - plants will flourish even if the tank is new.  There are a myriad of species available to you.

I would seriously consider changing the stocking plan in your tank; others will give you more info that will probably reflect this opinion.  However, it is your setup and your decision.

What species are the danios?  Some grow larger than others.  I would not be keen on keeping them in a 12g simply because they're extremely active fish and like a surprisingly large amount of space to stretch their fins in. ;)  Also, at their adult size the smaller danios will still grow to two inches or a bit more, so adding more would overstretch your bioload since you have the tetras in there too.

Black phantom tetras aren't large fish, but they aren't the smallest tetras out there either.  Expect two inches for adult females; the males will be slightly smaller and skinnier. 

So just by keeping the six fish you have, your tank is basically maxed out as far as bioload goes - and that's notwithstanding the schooling needs of the fish.

However, 12 gallons would be an excellent size tank to maintain a school of small tetras and a shoal of corydoras catfish.  You could keep 6 or so neon tetras, black neon tetras, glowlight tetras, or cardinal tetras, or even some of the smaller barb species in that tank along with three small corydoras.  Panda and pygmy corydoras are good choices.  These fish should be maintained in groups of at least three. 

How much light do you have over your tank (how many watts is the bulb)?  The species of plants you can support depends on how much light you have - some species require a large amount of light and others, such as the anubias, are happy with much less. 

If you go into the Invertebrates section of this forum and find the post entitled "My Shrimp Setup", you can see the plants I'm currently keeping.  I have four watts per gallon (high light), CO2 fermentation, and addition of fertilizer.

Some good plants for beginners are java fern, java moss, anubias, hornwort, and cryptocoryne.  All these are low-light plants and will grow in virtually any setup, so I'll mention them before you state what type of light you have.  Search them on the internet for pictures and more information.

Java fern is a low-light, hardy plant.  In fact, it tends to do better in low light tanks than in higher-light ones.  Some varieties grow quite large, but most stay within four inches in height. 

Java moss is, simply, an aquatic moss.  It also thrives in low-light settings and is slow-growing.  It can be trained to grow on driftwood, rocks, and even as a carpet on the bottom of your tank.

You have an anubias already, but there are many species available.  They come in many shapes, sizes, and even colors, and most are easy to keep.

Cryptocorynes can be difficult, but you seem well-informed and dedicated.  Crypts are beautiful plants that come in several leaf forms and colors; my Cryptocoryne wendtii is a small plant with gorgeous brown undertones that highlight the color of the substrate.  It dislikes being uprooted and repositioned, however, so save this plant f