When researching water quality standards, I found that a minimum and maximum was set for water hardness. Several sources also state that water can in fact be too hard. The question is how does hard water harm marine life? I understand that it is required to reduce the toxicity of Hydrogen ions and other trace metals, but cannot find anything concerning the effect of water that is too hard.
Hard water helps in direct and indirect ways.
Indirectly, hard water plays an important role in regulation of pH. Respiration and other physiological activities release substances into water that change the water pH and the ions that cause hardness act as buffers and help maintain pH.
Directly, they determine the ease with which osmoregulation occurs. The harder the water, the lesser the influx of water. So the fishes act like an open system and are influenced by the surrounding water.
There are very few fish who absolutely need soft water to survive. Since your pH and hardness are pretty low, it allows you to raise multiple fish from South America. You could do tetras, dwarf cichlids and New World cichlids, Plecos as well.
Potentially discus and altum angelfish as well if you're up to the huge challenge.
However, livebearers will not tolerate this type of water. That goes the same for African cichlids.
Otherwise hard water is completely fine for marine life and other aquatic species.
Hope this information helped!
If you look at the Wiki text on Hard Water, Marine hard water is not mentioned once. That's because it's a term used mostly to qualify freshwater.
Sea water doesn't vary alot in the oceans, only in inland seas and near estuaries and glacial melt, it's called brackish water, brine, not soft seawater.
Sea water which undiluted is called salty, highly saline, high salinity, water, not Hard Seawater.
Marine life isn't affected by the stone type mineral content of the seawater as much as by the highly reactive salts, both sodium and chloride are very harmful to life compared to most stones a the less soluble mineral salts.
Seawater contains a lot more calcium generally than most river water, even though some river water can be completely saturated in calcium and have very high PH. Some fish can adapt to a wide range of soft water hardness, and some fish can adapt to very salty water, but most fish are specialists who don't live in wildly varying ecosystems, only in the sea and in constant softness freshwater.
Read some aquarium chemistry experiments. they are aquariumabulous and will give you a lot of knowledge about marine biogeochemistry. It's pretty interesting to look at any pond and river and plant and know what chemistry it has, what it needs, and the processes changing it, it's quite simple to learn too, just learn some basic experiments and guides on water and pedology organic and mineral reactions and biology.