throw in a breeding pair of wrestling halfbeacks(brackish) and they will fight any other fish that gets near, you could do
1 pair wrestling half beaks
6 bumblebee gobies
and maybe 3 indian glassfish
but thats with brackish water, so only brackish tolerant plants. you could also do:
6 zebra danios or white clouds
1 hillstream loach
and maybe 3 otocinclus, but thats freshwater and with high currents and oxygen levels