I'm sure the plants could explain your water loss.
I'm not completely familiar with aquatic plants but I'm assuming, like any other plant, they "feed" on water. So, they are probably your "culprit." Of course, they make oxygen and help with the nitrogen cycle, too. So, I wouldn't get rid of them.
I don't have any live plants in my tank. Are they hard to take care of? I see all of the "crap" at the fish store to "take care of" your plants...everything from special filters, to Co2 tablets, plant "food", etc. I've wondered if you actually need all this stuff, or if it is just junk they try to sell to you.