55 gallon, and danios with rainbows too?? they are great
If you have all of those fishes, I suggest that you get some amazon swords, they are perfect for the tank. They grow up to about 30-50 cm tall, and they have tough durable stems, so you don't have to worry about having the fishes eat them, because they will grow fast, and they just need to have a lot of light, and some fish waste, because they usually feed on it.
Also, if you will be getting a pleco, they will eat them too. Wisteria, Cabomba, Java Fern, Anacharis, and Anubias, they will grow very fast and anubias plants will grow fast in low light conditions, since they are grown deep underwater where the light doesn't really reach the plants much down in south america. They grow a lot better in warmer waters, and softer water. They grow fast too.
Anacharis, they are also fast growing plants, and fishes love feeding on them. I have a few of them in my 350 gallon tank.
Basically it depends on what type of fishes you have, if you have fishes that don't really like to eat plants a lot, and don't bother with them, then you should get some slow growing plants. If you like to have a lot of semi-aggressive down to community fishes, they will eat a lot of the plant leaves too, so then I suggest you get fast growing plants. Dossile fishes are laid back, and are always calm, such as the lyre tail guppy, and some type of mollies, they will not be eating your plants a lot. They like to stick to frozen foods, or flake foods.
I am guessing that since you have a few danios, and rainbows, then you might want to go with some of the plants I suggested.
I hope that helps
