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Fish require very few trace elements and besides, the majority of their dietary needs are met perfectly well by any of the common flake foods.

If you're trying to address the needs of the fish, the GH isn't all that important. If you do want to supply Ca & Mg, why not use CaSO4 and MgSO4? These compounds dissolve pretty quickly and sulfate is pretty innocuous when it comes to both fish and plants. I'd add a bit of CaCl2, just for the calcum, and maybe a bit of K2SO4 for the K. Flourish trace doesn't add anything that regular Flourish doesn't. It's pretty much a product without a purpose as far as I can tell.

Honestly, if you're not trying to do something special, why not add 10% or 20% tapwater to your RO? That would be 100 times easier than messing with all the chemicals.

Don't forget KH. When it comes to fish, this is far more important than GH.
 

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I'm no fry expert, but my understanding is that they really get everything they need (including trace elements) from their yolk sack and the food that they'll eventually transition to.

What makes you think you need a GH that is so high? If the fish are really found in nature in a low TDS environment, the GH of that water will be very low too. The only thing that concerns me about what you're doing now is that the CaCl2 supplies tons of chloride - certainly much more than they see in nature. They need more than zero, but I really think CaSO4 would be a better option for the majority of your calcium.

Good luck!
 

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A GH of 2 or 3 is actually quite low, but there are plenty of natural waterways with a GH of almost zero. Fish live and breed just fine in these areas. I'm saying that if you're trying to keep your TDS low, you can get away with much less Ca & Mg, assuming you're only worried about breeding fish.
 

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Chloride is not the same as chlorine. Chemically they're night and day. In the same vein, sulphate is also not the same as sulphur.
 

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Re: You are asking way more questions than I can answer reasonably.

Believe me this is a very tough problem!
Yes, exactly.

I have a pretty solid chemistry and biochemistry background and I've developed a "recipe" for reconstituting RO water that seems to work in my tank for my needs. It's taken a couple of years to work this out and lots of thought and effort.

After all that, I'm actually still pretty clueless about many of the interactions between nutrients. A simple program is almost always better unless you're willing to go to the effort to understand the chemistry and follow trends in a scientific way over weeks & months.

For 90% of the people out there, adding a bit of tapwater to RO water is good enough - arguably even better than experimenting "in the dark".
 

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5% tap should be fine, assuming you're dealing with soft-water species.

If you use only enough Equilibrium to bring the GH up to 3 or 4, the effect of the potassium should be pretty much zero.
 
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