I’ve had a few people comment that one should not use seaweed extract in aquaponics systems because it would cause salt to build up and even heard it suggested that the occasional use of salt is defeating to the aquaponics.
I can understand some portions of the arguments but I have never seen an issue with salt building up in my aquaponics system. I have been running for a few years now and I’ll admit that most of my advise is really a conglomeration of collected wisdom from home scale aquaponics discussion and I don’t have any university scientific studies to back me up so take it as it is.
Anyway, I’ve added salt to my system generally only before adding new fish in order to protect them from nitrite spikes and handling injuries that can happen during initial stocking. I have salted as high as 3 ppt before and without doing water changes, only topping up, I’ve had that drop back down to barely measurable levels. Many plants will take up the salts in small amounts and the salts will be removed from the system slowly over time. So I definitely don’t agree that one should never salt a system but I now rarely go above 1 ppt even when adding new fish and that is generally a one time addition of salt perhaps each year and then I allow it to drop back down naturally. This kind of addition of very small amounts of salt are not going to build up unless your source water is also high in salt.
And on that same point, small amounts of Maxicrop or Seasol seaweed extract added once a month for the fist few months is not going to cause salt levels to build up in an aquaponics system either. Keep in mind, you are not meant to be adding one OZ of seaweed extract per gallon of water in the system, the amount of extract added should be based more on the growing surface area of the grow beds and how much seaweed extract the directions say to use to water that area. Example: if the bottle says to use one table spoon of extract per gallon of water and use that to water 100 square feet of garden, then if you had 100 square feet of grow bed, you would add one table spoon of extract to the system (these numbers are just off the top of my head as an example please read the bottle and do your own math for your system.)
Now how much supplementation one might need will be greatly affected by the source water, the fish feed, the media, the pH, and the maturity of the system. My Aquaponic system is heavily buffered with shells so I don’t need to alternate adding potassium bicarb and calcium hydroxide to my system so, I don’t get the extra potassium that way so I need it occasionally another way, hence why I still occasionally add seaweed extract. But once a system is nice and mature, most of the other trace nutrients that seaweed extract provide will become available from the mineralization of the solids in the grow beds so once a system is mature, the seaweed extract is generally not needed.
It is also important to note that seaweed extract alone will generally not provide all that is needed for the plants since the seaweed extract that is safe to add to a system cycling up with fish will generally not have much nitrogen content as that could over load a system so seaweed extract alone won’t replace the fish. It is simply a potassium and trace element boost for a new system, not something to be adding weekly long term.