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[AGA-Member] Magnesium nutrient issues

My 90 gallon tank (actual gallons about 76) has lots of long stringy algae,BBA, and one large old SAE.
As it's a tall hexlike diamond shape, I've got power compacts on it, 2 X 55 watt and 2 X 96 watts, for total 302 watts, or not quite 4 watts/actual gallons.
It generally has 0 nitrite and about 12 ppm nitrate, probably due to fish + food.
I'm not certain how many inches of fish it has because the Corydorus catfish breed in there, and I keep finding new babies.
It used to have CO2 injection issues, and CO2 was off completely for about 2 weeks.
I'm trying for a community tank balance, as I'm keeping both Cardinal tetras (soft acid water) and Cryptocorynes that like hard alkaline water (bad owner!!). In fact, the C. balansae grew better when my CO2 line was down.
I think I finally got the CO2 supply fixed this weekend, thanks to helpful advice from folks here.

Now I'm thinking about the nutrient dosing to work on that hair algae and BBA.
I'm coming up with some odd numbers on Epsom salts from the ratios that people suggest.

Normally, with CO2 injection, the tank was at pH 6.8-7.0, dKH 5-6, dGH 7-8.
Without the CO2, the tank went up to pH 7.4 - 7.8, same dKH and GH.
Tap has KH 2, GH 3, so some months ago I started adding powdered dolomitic limestone (from Greg Watson) and baking soda to bring makeup water one point each to dKH 3, dGH 4. This isn't as high as some advise. dGH - dKH = additional alkaline metals such as Na, Mg, and so on. One point of the difference is the soda I've been adding. Local waters are alkaline but not dolomitic, so I'd suspect the remaining point, from the source tapwater, is also mostly sodium.
Which says there's hardly any Mg in there.

Awhile back someone reminded me of Tom Barr's advice, and suggested bringing up dKH to 4-5 and add some soda for better pH buffering. Because of that, and to better match the tank's pH without any CO2 injection, on my latest 30% water change, I doubled the limestone and soda amounts as I got the CO2 injection going again.
Now, with CO3 injection started, the tank tests at pH 7.4, dKH 4, dGH 6.
The difference is 2 points, just what you'd expect from doubling my previous soda amount in the makeup water.
I also added fertilizers that include macros, Mg, Fe, traces, and so on, but clearly not enough Mg to affect the dGH-dKH hardness difference very much.
As I adjust the CO2 injection, I'll bring that pH down gradually to 6.8-6.9 to get better CO2 percentage.
But perhaps I ought to increase my limestone and Mg amounts too.
That made me start to recalculate my dosing drops.

Please correct me if I'm wrong, but could I increase dGH by skipping the soda and adding Epsom salts for Mg instead? This should increase general hardness for buffering the pH, and the plants would like Mg better anyway.
I thought of this because I saw comments noting that the ratio of lime to Epsom salts should be roughly 4:1, or:
CA(CO3) : MgSO4, should be 4:1, which ties my dKH to how much Epsom salts I add.

Working from "the ideal Mg amount":
My hardness test kit instructions show 17.9 ppm = 1 degree for both dKH and dGH.
Chuck Gadd's nutrient calculator page suggests the guideline of 5-10 ppm of Mg, but other comments suggest it can be more.
If I maintain the ideal Mg of 5-10ppm, that 4:1 ratio gives 20 - 50 ppm CA, or dKH =1.1 - 2.8, which I thought was pretty soft water. A lot softer than what I've got going!
But list comments note that many plants like hard water, that it can go higher.

Working from the hardness I've already got:
With current dKH of 5 - 6, or 89.5 -107.4 ppm Ca, a 4:1 ratio that gives 22.38 - 26.85 ppm Mg.
That is double the high recommendation. Also, it requires adding 4 - 4.5 tablespoons of Epsom salts dry directly to the tank.
This sounds like an awful lot!
It also adds that same amount of sulfur, and I planned to add sulfur elsewhere by using potassium sulfate (K2SO4) as a K source.
(I don't want to use KNO3, because the N is plenty high in this tank right now--I hope that it'll go down if I can get the plants growing better..)
Do we have any guidelines on how much sulfur is too much?
Also, how much change in dGH does that much Mg do? Don't think I'd need to add any baking soda, though!

Whic leads down to the final question, would it be okay to push up the Mg to keep the ratio with Ca intact, or should I leave it at Chuck's 5-10 ppm and a lower calcium amount and lower dKH, which I suspect the fish would like a little better?

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