Everyone will be getting tired of me, but as you've asked something that is right down my street...
The pattern on wild angels is the guide to their environment. They live amongst vertical plant roots, branches, and of course plants. Their vertical stripes allow them to blend in, and they live in large groups. They are not solitary fish in nature, same as Discus
. So if you can find branches, great. Have them sticking vertically up from the substrate, leaving some open area. Bogwood chunks representing fall trees or tree roots are authentic.
Around the sticks plant Echinodorus bleheri; in a 55g I would use three, not equidistant to avoid it looking contrived, as these plants will, given room and nutrients, grow to the tank in height and spread and be very obvious so have them spaced apart, maybe one slightly in from the left, one a plant width further in from that one, and one somewhere on the right side so there is different spacing between each and the tank walls. If you plant them close to the wall, they will tend to grow leaning; if away from the wall, they will be more symmetrical and vase shaped. Either is fine, it depends upon the effect you want. You can see both ideas in my SA tanks.
Another good plant is Echinodorus major
, the leaves grow quite straight up so it does not spread out as much as E. bleheri, and they have wavy edges. This plant if well nourished (root tabs/sticks do immensely help this species) it will grow to the surface and even beyond, leaves laying on the surface. A nice plant in a rear corner, there's one in my 115g in the back right corner, and one in my 90g just left of centre.
If you have slightly basic water (pH 7.0 to 7.8) Vallisneria is a super plant. The corkscrew species is a bit smaller than the large V. spiralis. Angels love to cruise through these plants.
In the foreground, nothing would beat pygmy chain swords in this setup, the lighter green and narrow leaves contrast well with the darker green of the afore-mentioned species while retaining a vertical appearance. E. tenellus is smaller, E. quadricostatus a bit larger.
Floating plants are recommended since angels do not like bright light, like all the forest fish. Authentic plants like Amazon Frogbit
work, or from India Ceratopteris (floating India fern or Water Sprite