algae

What causes algae? All my plastic plants were covered in a brownish algae. I
used an algae destroyer which did the trick, but I had to take out all my
natural plants to use it. I'd rather had not had the problem in the first
place. My fish love eating the plants and now they have no fun in destroying
them!
Also, one of my goldfish keeps chasing another. The other day I actually saw
it tear the others tail fin! Is it being frisky or being a bully? Any help
would be appreciated. I'm new to keeping goldfish. I have a 40 litre tank
with 3 smallish goldfish in it.
Jacqui