As simple as setting your Aperture up and down.cS said:How would I set up my camera so that it will focus on only a plant/fish/area and blur everything else (read: algae :mrgreen: ) around it?
f/2.8 - short depth of field (* face of fish in focus while rest of the body blurred)
f/11 - long depth of field (* face and body in focus)
Applied your aperture settings to your plant photography and you should get desired effect.
There is also good old PhotoShop. If you need tips, let me know.