Oh jeez... For some reason I thought the Endlers wanted to have a higher KH than my tapwater provides and I had this stuff on hand. But now I am not finding that information anywhere on google. It does seem that sometimes people refer to GH and KH interchangeably so maybe that's where I got the idea.
Yes, unfortunately a lot of online literature and retailers of livestock etc. are using the terms GH and KH rather indifferent. KH regulates the buffer capacity of the water - its resistance to change in pH (low KH means low buffer capacity thus low resistance to change in pH). If your KH comes from CaCO3 or MgCO3 it will impact your GH, if it comes from say NaHCO3 or K2CO3 it wont. Tropical freshwater fish do not "care"
directly about KH unless you're keeping fish that specifically require a stable above neutral (>7) pH in which case you also want a higher KH. Not common for a tropical planted tank.
One thing that comes to mind, that many feared in the past, is the so-called pH crash... I now believe that this was always a side effect of something else and catastrophic going on in the tank, rather than just the pH dropping. A well maintained and stable tank should never suffer from this. Think about it empirically: Hobbyists that inject CO2 into their stocked, acidic and extremely low KH tanks would be in trouble all the time with their daily 1 or more pH swings if this would be an issue.
Low KH makes it much easier to create a natural and acidic environment for the plants that promotes uptake etc. By far most plants will generally do better at low KH rather than high KH - and while a lot of plants, especially in the easy category
will do just fine at higher KH/pH, none - that I am aware of - specifically requires high KH/pH.
What is the danger of using this product?
With your 3-6 grams of Alkaline Buffer (NaHCO3) per 10 US Gallon your adding 20-40 ppm or so of Sodium (Na) which is of no good to your plants or livestock.
What let me astray years ago with this product was a misguided obsession about keeping a certain KH level and at the same time a certain stable pH level. I ended up with a toxic cocktail in my tanks that was part Acid Buffers and part Alkaline Buffers to target that specific KH and pH level. I do blame this in part
on the dishonest marketing by Seachem - prioritizing sales over the actual needs of the hobbyist.
fertilizer with so little calcium in it if this ratio is so important? I'm reading the label now and it says there is .38% Mg and no calcium at all, actually.
Calcium in meaningful amounts are not part of fertilizers as such, and most fertilizers only provide tiny amounts of Mg. The significant portions of Ca and Mg that you use for remineralization to reach a certain GH are provided with such off-the-shelf products as NilocG GH Booster, Dennerle GH+, Equilibrium etc. and usually premixed in a ratio somewhere between 3:1 - 4:1. With DIY dry salts such as CaSO4 or CaCl2 and MgSO4 you can get exactly the ratios and amounts of Ca and Mg you need/want and will cost you way, way less in the long run.