30 mg/L (ppm) is where my tap water is at
I went back to review before responding, and saw this was mentioned in post #1, sorry I missed it.
So, that is very soft water (my tap is even softer, around 7 ppm) and you thus have many options for a 20g long. Stay with smallish fish; these always look better in smaller tanks, and you can have more fish for the space, which makes a more interesting aquarium in the end. Plants would be advisable, but you have a couple options.
You could do a natural aquascape from regions that have very soft water, such as South America or SE Asia. Use wood, lots of wood, as chunks representing logs, tree roots, tree stumps (vertical straight chunks of wood are ideal for this), etc, and branches if you can find some. Sand substrate, darkish; I use Quikrete Play Sand which you can get at Lowe's and Home Depot for about $7 a 25kg bag, which is more than enough for this tank (it is helpful to have spare down the road). Collect some dead leaves now in the autumn; oak, maple, beech work well, generally any hardwood that is deciduous is safe. Collect them from the ground after they fall naturally, in an area free of possible pollution. Lay them out to dry, rinse them first if you like; avoid any with bird droppings. When they dry, store them in a plastic bag and add a few as needed. Once they waterlog they will sink and you can lay them over parts of the sand for a very authentic aquascape. Here, plants would be confined to floating only, and a good covering would dim the light and make the fish sparkle.
Alternatively, some moderate-light plants can work, like the pygmy chain sword. Floating plants again, fewer to allow more light through.
If you want more SE Asian, swamp or pond/ditch-look, again sand, lots of wood, floating plants. Some substrate-rooted plants that have surface leaves work well here, like the Tiger Lotus varieties, and Aponogeton. Floating Water Sprite is ideal.
Thinking of fish ideas, for the SA tank there are pencilfish, some of which are very peaceful, others less so.
Nannostomus marginatus (the lovely dwarf pencil),
N. mortenthaleri (Coral Red, a real beauty) or
N. rubrocaudatus (very similar) one of these in a group of 9-12.
N. eques swims at an angle is is very peaceful, a group of 8-10.
Hyphessobrycopn amandae (Ember Tetra) in a group of 9-12 would add an orangish-red colour. One of the dwarf cory species, in a group of 9-12 (
Corydoras pygmaeus, C. hastatus, C. habrosus). Not all of these together, but they are species that will work here.
For SE Asian, there are the small-sized gourami species, like the sparkling pygmy, Eyespot, Licorice. The dwarf rasbora in the genus
Boraras are perfect tankmates. Authentic substrate fish are more difficult, there are a couple of smaller loach species that are peaceful enough.
You don't want much filtration in any of the above, a simple dual sponge filter.
Byron.