hrm, i was hoping some wiser voices than mine would chip in, but here is my limited pool of suggestions! (i'm only sort of an oddball person.)
yes, there is the African Brown Knife, which is similar to the ghost knife, but doesn't get nearly as large. if i recall, it reaches somewhere around 12"--smaller, but a little too big for me to really feel comfortable recommending it to you. according to my readings, it also prefers soft/acidic water.
i personally have a strong bias towards keeping freshwater puffers
i have a South American Puffer (SAP,
Colomesus asellus) and adore him. however, most of the puffer species can be a bit tricky to feed. many will not accept processed foods (flake and the like) and all of them require access to "hard" foods in order to keep their teeth ground down. the SAP is especially notorious for needing dental work due to an improper diet.
SAPs are unique among freshwater puffers in several ways. First, there are only two fresh/brackish water puffers found in S. America:
C. asellus and
C. psitticatus. Second, there is some evidence to suggest that SAPs are schooling fish and can be kept in groups in the home aquarium. Third, it's one of the few species that will accept processed foods (some might even argue that its the only). They're also very active fish which need HEAVILY decorated/planted tanks to prevent boredom and the development of neurotic habits.
For a full list of freshwater puffers see
http/www.pufferlist.com/freshindex.htm
i'm also really keen on the various gobies & gudgeons available. there are plenty of smaller ones such as the peacock, the goo-obo, the bumblebee and the white-cheeked goby. some of them can be a bit tricky to feed due to their teensy size, but frozen food is generally a good solution to that. nmonks swears up and down that the bumblebee goby, if sold in freshwater, will do just fine if kept in hard/alkaline water like what you have.
A quick search for topics mentioning gobies or gudgeons.
This is a topic listing lots of oddballs suited to smaller tanks. Even though your tank will be closer to 30g than 20g, this topic is still a good reference. A lot of oddballs are territorial and a lot of them need extra clean water. An extra 10g will only improve your ability to keep them.