I've run into this problem on a couple lamps that I filtered because they wouldn't clear up. They're completely clear after filtering, but cloud up again over the next several runs. If I re-filter, the same thing happens again. I clean the filter using distilled water, so hard water shouldn't be an issue in this case.
Thanks for your replies, guys! I'm definitely going to try distilled water with my next attempt.
So regular salt (sodlium chloride) is clearer than epson salt - or what do you suggest instead? Any experiences with Mathmos Astros and gylcerin or propylene glycol?