Gti_Leo
Aquarium Advice Addict
This is a mystery that resurfaces about every 6 months. Some see this residue and some don't. I have also been thru hundreds of pounds of reef crystals and instant ocean before that. The IO did on occasion leave the type of residue described. I think it might be a bit of clay and inert. The RC has not left the residue for me, but many others have reported this residue. I don't think it has any negative effect other than needing to clean the make up barrel more often.
i've had it also it may only be an effect if it sits in a mixing bin for a while i tend to mix a batch and leave it for a few months with a powerhead moving things around and have noticed this crud on the surface if there is not much surface aggitation, when i put a bowl in to scope out a bit of water you can soemtimes see the stuff on it. but once its in the tank there really is no sign of it