my buddy has 8 of them in his tank.
he has a 8'' sand bed but in some spots it is only 1'' and some 4'' ect the eels are all over the tank some as low as 4'' of sand
its not the water level if it was it will fill up with water not foam.
if your skimmer is lifting the lid like the one in that pic you posted it is because the water is set to low in the skimmer and the foam is super dry and not much water in it to help it pop. if you look at the pic you can see...