Well, we do have a filter (little fluval-1, but it seems to be doing ok in the 15 gal) & we're keeping the water pretty clean - doing 3/4 or so changes 2-3x's each week & bottom-suctioning daily since mississippi maps are sensitive to dirty water. The tank's not near a window, but there is a bit of surface alge forming (don't mind that though). Obviously the temps are warm - 78-79, so that may be it, Marisa.
Right now we're out of dechlorinator, so that may be an issue too - didn't even consider that possibility. I'll try again next week using chlor-out. I was suprised at the water lettuce going downhill so quickly though since it takes over EVERYTHING in the pond. I have to weed it out several times a week to keep it in check - water hyacinth was just as bad when I had some. Guess indoor tanks are another story for plants.
I'll post when I get more & try it w/ dechlorinated water. Also, that little filter makes quite a current in the small tank, so maybe between that & Jimi climbing on it was just too much for the plants.
Does lack of aeration (bubbles) matter? We don't have feeders or a pleco in the tank yet, so haven't gotten around to an air pump, but plan on adding one soon. I wouldn't think it would matter to a plant though...

Thanks for the ideas all!