Salt Gravity

[Moogie]

Hey there, I'm new to this forum. My boyfriend and I are setting up a tank for GSPs and I was wondering your thoughts on its specifics.

We've fully cycled a brackish water tank:
15 gal., 0 nitrate/nitrite, pH 7.5, water 82 F
stocked with live rock and plants

The problem is that he and I disagree to the SG level. I've been seeing on this forum that people keep their salt gravity at around 1.001 to 1.005 which is what I'm in favor of; however, my boyfriend wants to keep it at 1.010. Which of these are correct, does it make a large difference?

I had a GSP which recently died in the tank and I believe it was due to a high SG. His color became muted and his black spots turned gray with fuzz(still have no idea if this was a parasite...). His stomach also seemed distended.

It has been three weeks since his passing and we have done two big water changes, the tank is fully cycled and we have a temporary Dalmatian Molly in there. I have three questions:
1)If that was a parasite that killed my last GSP, is it okay to put new GSP in the same tank after cleaning it
2)was it the high salt gravity that killed the poor fish
3)what is the proper SG for brackish water?

[Pufferpunk]

Start with this article: library/puffers-in-focus/an-introductio ... d-puffers/

[Moogie]

thanks @pufferpunk