is there such thing, as too many plants?

Yes, I think so. If your fish are unable to "stretch their fins" and motor about the tank properly, or rather, if there's not enough swimming room, then yes, you may have too many plants. ;)
 
I agree with Harlock - it's important to leave swimming room for fish. As well, unless your lighting is very powerful, when plant growth gets very dense, light to lower stems and leaves gets blocked resulting in a lot of dropped, rotting leaves. There's a fine line between 'heavily planted' and 'jungle' :)
 
I must say that floating plants are AWESOME... I've noticed a big difference in the activity of my fish since I put in some wisteria (eh, not a real floating plant, but it does the trick). The cories and pleco are a lot more active and outgoing in the tank with the shade.

the fish really seem to appreciate the dampened light and the stimulation that comes with a whole ton of floating plants. They're also a lot easier to keep alive than some of the others I've tried in 2w/g lighting w/o CO2 injection. Haven't had a spot of algae since then either.

Wisteria is my new best friend until I get a CO2 system.
 
So many people HATE duckweed, salvinia and fairy moss, but my tanks ALL have it in mass quatities because I keep mainly apistos and SA fish. They all love it. They're more secure than they were before I started using the floating plants too.
 
AquariaCentral.com