HI Mcminnj. Plants are contributing to your tank's ecology in many ways - that is, they're very beneficial to have. They use up nutrients from the water that otherwise would be left for algae to feed on (they sort of compete for nutrients with algae and that's why they prevent algae from growing all over your tank - if you have enough plants). They consume ammonia and nitrite - very toxic substances to fish - as well as heavy metals that are also toxic if present in large amounts. They also help to keep your pH stable and produce oxygen which is necessary for fish to breathe. I don't think plants have any negative impact whatsoever - unless they are all terribly rotting and thereby producing a lot of acids that would lower your pH a lot - but that is not necessarily possible since you'd know to remove dead plant matter if they indeed were rotting.
If you're planning to have plants in your tank, that's great. Just remember that to thrive, they need appropriate lighting. So before you buy any plant do your research on it to make sure you'll be buying something that will grow in your tank. If you have standard lighting, don't get any plants that have high lighting requirements. Good low-light plants are Anubias, Java Fern, and Java Moss. From my personal experience Water Sprite grows well too under low light. I think so do various Vallis species.
yeah I'm glad I put live plants in2 my tank this time round I seemed 2 go 4 1s that would not give off leaves 2 float about constantly I went 4 larger leaved plants and think stemed plants it worked well plants r not harmful 2 ur tank hes tellin porkies!!!! lol
lol no offense but that was a pretty generic question... pretty much they do what everyone has said, they look nice, give your tank a natural feel, oxygenize the water, provide shelter for fry, provide cover for smaller fish, remove nitrates, and so much more