What causes this?

[CalmSeasQuest]

It's likely not related to your RO (that would be a ton of particulates to get through an RO.)

I find that the longer the freshly made saltwater is in the mixing bucket, the greater the amount of residue that accumulates on the bucket and power-head. If it mixes for only a few hours , I don't notice any deposits. Overnight or longer is when it appears. I went a few days prior to using the last batch of RC I had made and had significant deposits - so much so, I had to disassemble the power-head to get it clean.)

[BeakerBob]

It's likely not related to your RO (that would be a ton of particulates to get through an RO.)

I find that the longer the freshly made saltwater is in the mixing bucket, the greater the amount of residue that accumulates on the bucket and power-head. If it mixes for only a few hours , I don't notice any deposits. Overnight or longer is when it appears. I went a few days prior to using the last batch of RC I had made and had significant deposits - so much so, I had to disassemble the power-head to get it clean.)

This is fairly common and will happen more often if the water is really cold. If the TDS is good on the RO system, you do not have to change the cartridge. This precipitation/salting out of the minerals from saltwater preparation will not hurt the tank or the livestock. As CalmSeasQuest has advised, you may have to clean out your mixing pump. This stuff should dissolve easily in a acidic (vinegar) solution (or diet coke ).