doing more water changes doesn't compensate for being over stocked. being over stocked will cause the fish to be stressed and there frankly is not alot of room in a 55g for a bunch of fish.
how do you get the idea that a tank that is 48" long and 12" wide make a suitable home for a 30+" fish? instead of buying a bunch of fish that dont work, do research and use the search function for appropriate stocking lists and experiences from other keepers with similar fish.
what is your stock right now in the 55g? your idea of 1 blue acara, 1 sajica, 1 rainbow cichlid, and 1 firemouth with a poly (either senegal or delhezi) is a good stocking list. when you get multiple fish that get 6+" in a 12"wide tank you start to get crowded real quick
i had 6 fish in my 55g at on point and it was very crowded. most were about 4". i couldn't imagine them all in there if they were any bigger. i now have a breeding pair of severums in there and have a 75g sitting of the floor waiting for them because its getting too small for them.