How thick is the petroleum jelly barrier you are putting on there? I have had my hissers in a 40 gallon aquarium for about 2 months with a 3 inch petroleum jelly barrier at the top, and none of my small nymphs have escaped. I have done the taping the sides thing before when I had them in a different enclosure. How far do you have the petroleum jelly below the openings? Sometimes my nymphs will climb to the top, but turn around when they get to the barrier.