I have a 20 gal tank in a window that is fed from my sump, and drains back to it as a freshwater refugium.
I use papyrus for plants, but have also used water hyacinth, cryptocorne, java fern pothos, and whatever grows well in it. I believe the kind of light you use, will be the determiner of what kind of plants are for you.
For animals I use rishi and gammarus shrimp, snails, and sometimes the fry of whatever fish is spawning at the moment.
Don't know if you are doing fresh water, or salt, but clams are a bit iffy in fresh, if they do not get enough food, they easily starve unseen (because they tend to bury themselves) and can quickly and easily foul the system.