Adding MG Supplements to "Fresh" Saltwater

popalud · 2026-09-05T13:21:59-0400

Hey all! I am getting my parameters dialed in but my tank water is different than my freshly mixed saltwater, which results in some swings following weekly water changes. I'd like to minimize those swings. I use MT Classic Reef salt which is lower in Mg (~1165) than I run my tank (~1350). Are people adding supplements to their fresh mixed/stored saltwater so its ready to go at water change time, dosing the water used for changes at the time of the water change, or after adding to the tank? Curious if there's any data related to each option. I have AFR and Brightwell Magnesion (liquid) that I would like to add to my batched saltwater (~25gal) that will stored about 1month before a fresh batch is made, but not sure if doing so will result in wasting supplements.

Saltyreef · 2026-09-05T13:25:37-0400

A lot of people suppliment their salt mixes to match tank parameters.
Though a better option would be to find a salt mix that is elevated to your liking already.

Randy Holmes-Farley · 2026-09-05T13:56:17-0400

Why do you think the salt mix is that low?

I would not add AFR to new salt water. The magnesium is ok to add, but I suspect you do not need it.

popalud · 2026-09-05T14:14:51-0400

Randy Holmes-Farley said:
Why do you think the salt mix is that low?

I would not add AFR to new salt water. The magnesium is ok to add, but I suspect you do not need it.

MT lists a range of 1210-1310Mg, 9-10dKH, 380-410Ca . I've tested my fresh water and got 1165 (salifert) 8.9(hanna), 400(salifert). Not sure how long the batch had been sitting but I'm making a fresh batch today and will test again tomorrow prior to additions.

Randy Holmes-Farley · 2026-09-05T15:23:14-0400

popalud said:
MT lists a range of 1210-1310Mg, 9-10dKH, 380-410Ca . I've tested my fresh water and got 1165 (salifert) 8.9(hanna), 400(salifert). Not sure how long the batch had been sitting but I'm making a fresh batch today and will test again tomorrow prior to additions.

I do not think home magnesium test kits are accurate enough to guide dosing. It is more likely at their specification, or close, than your kit is to be right.

This has more:

https://www.reef2reef.com/threads/just-say-no-to-magnesium-testing-rmm-is-born.1017528/