Yup it's only in the fuge. Have thinned out the cheato. Did a 5g wc today. See if this makes sense. I'm using a pre-skimmer box to help keep the DT's water surface clean. Otherwise the surface holds an oily protein. So the water being removed from the DT is mainly surface water. I am using a nano skimmer (not happy with it either) which is placed in the fuge.

What I'm thinking is that the protein being skimmed off the water surface is to much for the skimmer and cheato to handle?