My best recommendation would be a wartskin angler, alone. A warty will set you back at least $150. diver den wysiwyg will set you back like $200. The other anglers are too large or the pygmys are more expensive and very difficult to care for, if not impossible for some. A 20g would be too small for any fish that the angler wouldn't eat. I really don't recommend a blue spot puffer with any angler or sedentary ambush predator, they are notorious nippers and would lead to an injury that most times turn into a bacterial infection and death. Blue spot puffers wouldn't really be happy in a 20g, and would likely get nasty.
Borderline, as I would want a larger tank, borderline maybe an Hawaiian leaf fish. Properly sized a leaf fish and a warty could live together, The leaf fish would need to be larger than the angler as the angler could eat the leaf but the leaf fish could not eat a rotund angler.
A yellowspotted scorpionfish, these guys don't come around that often, and are super cute and have a very cool personality. Stays small enough where you could even include a hawkfish(properly sized larger of course) and even a leopard toby puffer. Even though the 20l would be borderline small for this fish, they are a safer bet not to nip. 20l will also be borderline small for a hawkfish.
Yellowspotted Scorpionfish - Sebastapistes cyanostigma (thatpetplace.com) A leaf fish could also live with this fish, but I wouldn't trust a warty. A flame hawk could live with a leaf fish but a longnose would be a risk because of their body type.
A 20l is too small for a fu manchu, I recommend at least a 40B for this fish, and that's bare minimum. These fish do like to swim but sadly too many people cramp them in tanks too small. A toadfish is out of the question, the common purchase size is already too large for a 20l, and they grow fast.
Please click on my name and "find all threads", you'll find tons of information on predators.
