Sure you can do a 55 aggressive. For example, I have a 45 gallon with a Toby Puffer, a purple Psuedochromis, a chocolate chip star, and a giant snail. I probably have room for one more fish such as maybe a longnosed hawkfish, a damsel, or a smaller wrasse like the sixline or Ornate. You could also do a dwarf lion and/or a small eel like a snowflake or zebra eel. You could also do a species only with an undy, a humu, or a frogfish.