The plecos could definitely be going to town on the sword plants but the light is low in the 50 gallon tank and quite high in the 10 gallon tank. Try having both lights on for 8 hours in both tanks for a couple weeks to see if the plants perk up. Those root tabs should help a lot too.
Put a slice of zucchini or cucumber in the tank for your plecos, they love the stuff and might leave your plants alone. I make a shish kabob with a metal skewer and the ends of veggies but pushing a fork through holds it where you can find it later too.