I would say the salt creep is responsible for the drop in salinity. As for needing a fourth channel on your doser, you should be able to combine the same components in your 3 chambers as is done with Balling, but using your own ingredients. IMO, no need for a fourth chamber. There are some additives, such as iodine or amino acids, that shouldn't be combined with anything you put in the first three chambers (I might use it for vinegar dosing). If you wanted to dose any of these 'other' supplements, channel 4 would be appropriate for this. Barring this, I would leave this channel unused and in reserve in case one of your motors or heads has an issue; you'll have a backup. Just my $0.02
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Dan