Mine has fine, tight mesh. No fine particles that I can see make it back into the water.That's a very harsh thing to say. Mine had nothing to do with being lazy. I got it when I had juvies I was growing up. Tank got 50% wc daily & at their very youngest it was twice a day! The eheim sludge extractor was for in between those daily changes, since juvies have multiple daily feeds & need clean water.It works for the lazy ppl that dont want to do water changes but once you see it work omg... your tank looks horrific for a good 2 hours after
I previously had a cheaper non eheim version, and it was crap & did a crap job.
I agree they're not for sand. I burnt one out using it over sand.