Too much light can bleach leaves - it happens particularly with Ceratophylum
demersum - the whorls of leaves become yellowish-brown in colour and have a
bleached look (completely washed out). They look like they are dying off.
You may not be burning the plants but you may be causing them to lose the
chlorophyll that they need for energy an give the plant its green colour.

Try switching one of the light off especially where the Hygrophila is
(probably background) and see if the plant responds differently.

Martin