The Wartskin Angler (Antennarius Maculatus)
Here's a fish that is perfectly suited for smaller aquariums, as it lives a very sedentary lifestyle, and in a perfect world, wouldn't really move at all if enough food came its way. Wartskins are intelligent and will learn to recognize their aquarist as a food source. They readily segue onto frozen foods, though you may find yourself offering live foods occasionally, just to see the Angler go fishing for its prey. There are few, if any fish that can rival the Wartskin Angler for pure visual interest.
Keeping anglerfish isn't all that daunting; they like irregular but large meals (think silversides, krill etc). They need something to hide in / on / around, and they need good water quality. Honestly of all these, the water quality can be the most challenging, because they do produce a good amount of waste from the feeding style they like. They also don't like a ton of light and dimly lit aquariums are probably best. They are best kept as an only fish in the aquarium, as they will eat fish smaller than themselves, and can get picked on by better swimming, bigger, more aggressive fish. They pretty unlikely to bother corals intentionally, but they do perch on things, and can get stung by things, so pick corals wisely if you choose to have some with your angler.
The Orange Spotted Blenny (Blenniella chrysospilos)
The ubiquitous Lawnmower Blenny has a better looking cousin that, somehow, everyone forgets about. The Orange Spot (or spotted) Blenny is the same fish husbandry wise, don't need a ton of space, love to have lots of algae to graze on, easy going with tankmates (except other blennies, or fish that look / act very similar). The difference here, is the brilliant orange freckles this fish is speckled all over with. They are gorgeous, they take well to captivity and they are great at keeping a bit of hair algae in check.
Keeping B. chrysospilos is pretty straightforward, but be warned that they won't live on algae alone. Supplement their diet with a high quality marine based frozen foods and when the algae in the tank runs thin, also feed a natural dried algae products like TMC's Algae Boost and Nutramar Nori / Ogo.
The Splendid Dottyback (Manonichthys splendens)
Dottybacks have a reputation for pugnaciousness that many aquarists frown upon, and as such, are often sadly overlooked when picking aquarium inhabitants. The Splendid Dottyback has earned its name and is truly dazzling to see in person. They are an amazing aquarium fish, that doesn't need tons of space, are the picture of durability, and are active all the time, quickly acclimating to most aquarium systems. They will fare well in larger tanks, with bigger fish, and are the perfect “feature fish” for that tank down to 25 gallons or so. In tanks this small, they should definitely be kept singly, or with other aggressive fish that can hold their own against this semi-aggressive beauty. A local fish store here in LA had one in a display with tons of open rockwork and a bunch of Azure damsels and it was gorgeous. Splendids are reef safe, being unlikely to bother sessile inverts, but may consume small shrimp and nip clam mantles.