I currently have a 20 gal unplanted freshwater tank. I have some plastic plants, but was reading that my molly might like some Vallisneria.
(1) Can I put plants in an already established tank?
yep
(2) What are the basic pros/cons of planted/unplanted?
see below
(3) Can I have a combo of fake and real plants?
yep
(4) Is there a certain amount that I should have?
A lot. LOL.
plants consume ammonia (and then) nitrates, phosphates, carbon dioxide and return oxygen and fish food.
So basically the recycle fish wastes into fish food.
Or from the plants perspective, The fish recycle plant waste into plant food. :shock:
You get the idea.
and in the process the entire tank becomes much more stable in operations.
Say something goes bump in the night and ammonia jumps up.
the plants immediately start consuming that ammonia over nitrates in the tank.
Preventing dangerous parameter spikes and preventing tank crashes.
then when things return to normal the plants go back to consuming the requlting nitrates.
Plus a heavily planted tank can become a net consumer of carbon dioxide and producer of oxygen each 24 hour period.
So basically it's a win win win all around situation.
I start a 10g with 4-6 bunches of anacharis, 4-6 vals, 4 small potted types and a single amazon sword.
Actually that would be good for a 20g as well.
For an existing tank, I would drain the tank and save the water and fish.
The add substrate and plant the plants.
And refill the tank with water poured over a dish.
for a substrate I use 1" peat moss (1'x1'x3' plastic cube $10 or so), 1" play sand (50 pound bag $3), 1" pro choice select ($7 for a 50 pound bag) (can substitute gravel), each layer wetted and leveld then the next layer added.
then don't add any food for a week so things can settle in.
my .02