There are many knifefishes that would suit your tank and only grow to around a foot,the african knifefish Xenomystus nigri will only attain sizes of 12" .This fish however is supposedly very difficult to care for (ive never kept one so i cannot confirm) needing very soft acidic water with excellent water parameters.They are also extreemly predatory and can take quite sizeable fish.
Another common example is the Asiatic knife fish Notopterus notopterus,more peaceful than the african knife and less demanding of water conditions but still only attaining a adult size of 14",this fish like all knife fishes is still a predator so tankmates must be quite large.
There are also many south american species which wouldnt outgrow your tank but these are very delicate and rarely seen in stores.