Hi all,
Looks like either potassium or nitrogen deficiency. I'd try adding some KNO3. Could well be snail damage as well. If you turn a leaf over, and it is snails, you will see hollowed out bits in the aerenchyma "float" cells.
I always know if any baby pond snails (Lymnaea stagnalis) have crept into the tanks (I added them to the water butts and Daphnia buckets to graze the biofilm), because they eat the floats on the Frogbit.
cheers Darrel