i think a 55 is too small.imo a 125+ would be the minimum .angels can be mean to each other but i am not saying it wouldnt work depends if you add them at the same time and they are both the same size.it might be ok.i would try it and have a back up plan to remove one if it dont.i have had 5 tangs in a 150 gallon with no problem but it dont always work.