In addition to what Seachem says about their product, it also works very well as a mild algae-cide. If they marketed it as such they'd have to jump through a bunch of hoops and regulations, so easier to say it is just a source of Carbon (I was lucky enough to talk to a Seachem scientist at the AGA Convention last year - man, Excel is COOOOL stuff!).
a formaldehyde type chemical that provides a source of carbon, like carbon dioxide, for the photosynthesis process. Mostly used in smaller, low light tanks due to it's cost and the relative ease of a yeast/sugar DIY CO2 delivery of carbon.
Tom almost explained how the plants pull carbon from this chemical.. I was really hoping to hear it! Jen's right, really cool stuff.
It definitely melts Anacharis and Vallisneria, some types of moss if OD'd. I've noticed that it mostly affects plants that don't have an emergent form. I'd be very surprised if it hurt your Amazon swords though. How much did you put in?
If you've just recently obtained these the amazon may still have emerged foliage. Once planted they will always grow a new type of leaf for under the water - submerged; then the leaves which were emerged grown will be discarded by the plant.
They are old plants that have been in my tank for a 7 months. However they were significantly weakened by a severe algae outbreak. I guess plants that were on the verge of dying, shouldn't be exposed to 2X excel treatment
Your swords should have a strong enough root system to regrow new leaves and fully recover from the Excel O.D. Just don't pull them out of the substrate, leave them as is. You may carefully prune away the old leaves if they start to deteriorate.