the general rule for stocking is an inch of fish per gallon
I have a couple of danios and I've read they like to be in large schools so maybe add some more of them, I know some can have great color but it can depend on the lighting you have as well, mine looked pretty dull till I got a bulb with more red and blue in it and all of a sudden they looked more colorful and you could really see the patterns aswell. I don't know much on cory cats as far as the coloring that the differant species have, I do know both danios and corys are pretty mild in temperment, the corys will like to be alone or in groups and can be very shy and not like other fish being near them, the danios/minnows and be aggresive but thats because they are a very ative fish if you have enough of them they will be mild towards other species and the aggression will begine to look more playful with eachother. you could try a pleco if it's one that only gets to around 4 inches but it may need a bigger tank at that size depending on if your tank is a tall or a long 20g and how much hidding space it has also plecos tend to be more active at night, you also have a decent amount of fish in the species you have so a fresh water shark would probly leave them alone. your best bet is to look up differant kinds of fish look see what fish will fit in your tanks peramiters best (temp, lighting, overall temperment, average ph, amount of open space vs. hidding area) keep in mind some fish prefer plants and like to eat them, I find live plants are easy to care for don't add alot of maintaince time and look better then fakes, here is a site (posted below) I use not to buy stuff but they have fish seperated out into familys and show pics of them and tell you what kind of enviroment they like, I find a family I like and then look for sub species to see if there is a coloration I like best and if there is a certain one that will fit in best with my tank. hope this helps some, let us know what you decide to get.
Aquarium Fish: Tropical Freshwater Fish and Saltwater Fish for Home Aquariums
