I live in Seattle and here we have extremely soft water, probably softest in the nation.
Tap water reads around 6 PH and 0-5 KH.
I have heavily planted tank and plan to keep Amano shrimp, Nerite snails and small fish like tetra.
Can they survive? or should I add crushed corals / kh buffer?
Shrimp and snails would benefit from having the pH raised. Their shells and carapaces will deteriorate otherwise. You can supplement their diet with a piece of cuttlebone. This is an instance where a bit of marine (Not Aquarium ) salt would be a good addition.
Wild caught tetra would love your water as is. Also most South American fish.
Do you know if their 7.0 neutral regulator will bring up the alkalinity much at all?
My kH is consistently low & my pH varies from tank, as does the gH, but they are generally not low.
(I freaked out when I found one fish in a gH of 27, until it realized that it was a constipated betta I had treated with epsom salts. I gave him a 75% water change. He's feeling better.)
I was going to go experiment with this tonight. Somehow I can't imagine I need alkaline buffer and neutral regulator too.
(I've got six systems involved now, from gH=2 to 18.)
BTW, I have been using the Weco wonder shell as a buffer and I've been using Straight Epsom salts and sodium carbonate to bring up the hardness. Something's not working and I'm thinking of doing some dr. Frankenstein electrolysis experiment on my aging water to see if I can get some kind of ion Exchange.
Do you need to bring the KH and GH up before you add the buffer? I'm thinking if you have extremely soft water and add that buffer it will vanish leaving you without much buffering capacity left.