What kind of fish should i get?

i have a freshwater 20 gallon long with 6 neon tetras, 2 cory catfish, 2 african dwarf frogs and an otocinclus. everybody gets along great and i have a very good feeding strategy for the frogs so they always get the right amount of food. (african dwarf not clawed so they wont eat any fish so please don't state that). i want to get a fish that has that shark look too it that will grow around 5 inches or less. and that will "monitor" the tank as the "big guy". i was thinking between a Betta or a Red Tail Shark. the tank is planted with buds that should be germinating soon. 4 grown bamboo plants a moss ball driftwood and 3 caves. making more caves won't be a problem. I'm good with taking care of my tank and i plan to still get better. also i can't upgrade to a bigger tank (said the landlord). please help me out i want to complete my aquarium. also has heater, and filter and hood with lights and all that god stuff.