a submerged heater shouldn't get hot enough to damage plants. mine never have, including corkscrew vals. but I've also only used quality heaters. i'm sure cheapy heaters might get hot enough to upset a val...I've just never seen it.
you have enough light. how about CO2? is your water on the soft side? Vals need a little hardness in the water, mainly for the calcium.
also, if your vals are all tied together with runners, snip them. any time a runner has 3 or more plants on it, split em. when they're conjoined like that, it taxes the original plant..the 'mother' plant.