not enough current with that much light maybe? plants burn up all the CO2 and nutes in contact with them, but without a way for water to circulate, they might not do so hot... all my plants outside pretty much ended up growing into one giant floating pad in one of my tubs... i tossed in some flame moss... bad idea in the sunlight... looks so gross... but there are sprouts of ludwigia poppin out, a crypt ended up out there some how, water sprite tries... but never lasts out of water..
in that tub, an airstone moves the water; but, nothing is technically planted, so dissolved CO2 is not an issue, all the plants have access to atmospheric gasses. (bad sentence.. but i'm not a grammar nazi.)
+1 . Water movement will help cool things off.