your tank is severely overstocked. while your fish may be "getting along", they are not going to thrive for very long. fish maintained in tanks which are too small or in conditions which are too crowded, develop severe stress syndrome. these fish do not exhibit proper color, do not develop proper fin form, do not exhibit proper metabolic development, do not exhibit normal behaviour, do not develop proper musculature, do not develop properly functioning organ systems and as a result, slowly lose their inherant resistance to disease. this results in a significantly shortened lifespan and along the way, lots of diseases for which the poorly conditioned fish is a good target.
twenty gallons is far too small for even one severum let alone the rest of the fish you have. a single BP might do reasonably well with overfiltration and weekly 50% water changes but your headed down the wrong road here.