I have Java fern attached to driftwood in two tanks: it grows fine in my 20 gallon tank, but in the 30 gallon, the leaves turn black or yellow and get holes in them. If I move the sick plants from the 30 gallon to the 20 gallon, they start to become healthy again. I have no idea what could be different enough between the two tanks to cause this. The 20 gallon actually has less light - 30 Watts per gallon - than the 30 gallon - 60 watts per gallon. I maintain both tanks the same way and use the same plant food in both. What could be causing the different results for the Java fern??