I use the light defuser because, I find that after about 60 days of being in 88-89 degree temps with little to no movement the water gets a little funky smelling, you know that how stagnant water smell. Also I'm not the most graceful hippo in the pond so sloshing is a given.

Plus I've noticed that the hot substrate helps regulate the internal temp of the box, as it doesn't lose heat as quickly as plain water, so temp swings aren't as big of a concern.

But thats just me.