ill do my best w/o knowing stats, tank, and fish
a mature tank means the water is always going to be "up to par" as long as you keep up the wc's and theres no crazy situations
most mature tanks (at least mine) will last for years with no problems as long as weekly/monthly maintenence is done
tank being "up to par" with a cycled filter means the fish are happy, and are use to their water conditions
fishies that live in good water parameters are going to act normal (which is a start)
tanks having more hiding places and or more natural, will make the fish comfortable
if fish are comfortable and happy, theyre more likely to show off their natural color
if there are no predators or bully fish in the tank they will be less stressed
fish that are stressed tend to turn bland colors. im assuming this is what you meant by brown
the other side of coloration
give your fish many different food options. i usualy mix color enhancing foods with healthy foods. usualy flakes because mine are community. fish that get their veggies are going to be healthy and show more natural color. i think algae wafers are the most universal veggie consumed by the majority of fish. my predatory fish eat wafers, my community fish eat wafers, my betta eats wafers, my shrimp eat wafers....but only my plecos and gouramis eat cucumber or zuchini.
add another type of food for variety. use treats everyonce and a while. i feed my bloodworms every now and then which (in my guess) add proteins, oils, certain other minerals and such to their nutrition.
a fish that is brown isnt happy and a happy fish is all of the above
but really stock and stats would be good too
