Lots to comment on. First, on the tank size...when I was considering a 20g long, I decided on the 29g because it is the same footprint but you have the extra height. Most of us lean toward overstocking, or always want more fish anyway, so the 29g is probably the better option of the two. However, it you had definite purpose in mind, say a group of Corydoras or a tank devoted to a pair of dwarf neotropic cichlids plus some upper dither fish, a 20g long might work. Though I would still go with the 29g. In a fish room with multiple tanks, a bank of 20g long tanks for breeding this or that species makes sense, but for most home aquarists the 29g is likely going to be more servicable. I have two of these, I really like them; they provide good options. My other favourite is the 40g breeder which gives you the additional length (36 inches/90 cm) but also the additional width (18 inches/45 cm).
Zebra danios should be in a 3-foot (90 cm) length tank, they are very active swimmers.
To the no-filter tank. Obviously this is possible, but keep in mind there are some mandatory factors. First, a sand substrate is better than gravel because it provides more surface area for bacteria. Second, live plants must be included, both substrate rooted and floating. Third, don't push the bioload with more fish than the system can handle; it is taken for granted that weekly partial water changes of 60-75% of the tank volume will be carried out. I had a 10g set up like this as an experiment for over a year. I had no light (it was in front of a west-facing window in my fish room then) so the plants grew to the back of the tank, and it was frankly difficult to really observe the fish with no overhead light. I would now and then place the overhead light on the tank, and discovered that the water was never crystal clear but always faintly hazy. After the year, I moved the tank back into the room, added a light and a single sponge filter; it was frankly better that way. But understand this is only "better" from my perspective of observing my fish; there was nothing biologically wrong with the system or the fish without a filter. Filters primarily keep the water clear, which is not the same as clean (how often have AbbeysDad and myself made this point?) and there is no reason for a filter other than clearer water, provided the tank is biologically balanced to begin with--and every tank should be or you are always living on the edge of a possible disaster. When my power goes out (very rare, fortunately) I never worry about the filter beecause I know it is not "mandatory" to begin with, but I worry about the heat as a rapid cooling can harm fish and even kill them.
It is often suggested that Zebra Danio must have a decent current from the filter. But this is in error. Engeszer's 2007 survey of the habitat indicates this fish inhabits calm, shaded areas in streams and smaller rivers, moving into seasonal tributaries and pools to spawn during the monsoon season. The water flow in small streams and rivers is not a raging torrent.