Swordtail Oddness

[ADMWNDSR83]

Some back story first.  I had three golden wag swords, a male and two females.  I noticed the male seemed to always be hanging out with a single female, and the other seemed to be an outlier.  She eventually began lying on the bottom for several days, except during feeding when she was in the fray and acting entirely normal.  All other fish in the tank were normal, and water parameters were fine.  After several days of this, she was stuck to the filter intake when I got home.  After an acceptable period of mourning(for those 30 seconds I was inconsolable), I went out and got myself a new female golden wag.  After several days in the tank, I'm seeing the exact same behavior beginning with this female.  What am I missing? I thought having multiple females per male was a good thing, and they would enjoy the school dynamic?  I am testing the parameter like they are going out of style, and nothing seems out of sorts.  All the other fishies seem happy and healthy.  Thanks for any insight!  

[Cory]

Lets start with the tanks water parameters? Especially pH, hardness and nitrates. 

[ADMWNDSR83]

She actually died as of last night.  There was a thick pink curl that appeared out of the anus, which then turned white, and she was lying on the bottom, then was dead.  All of my other fish are swimming around healthy and happy.  In case it was something with the tank, this morning, which by this point isn't affecting her anymore, my tank was at 0 ammonia, 0 nitrites, <20 nitrates, and pH of about 7.  The two things that do concern me about the parameters in general are a relatively low alkalinity (About 40), and very hard water (between 150 and 300).  Not sure what to do with those.  I've dosed with EasyBalance from Tetra, but didn't see much of a difference.

[Cory]

Well swordtails are a hard water fish, so you'll want to keep the hardness. pH being "about" 7.0 could be a bit low, Ideally it'd be 7.5 or higher for swordtails, but 7.0 is considered the bottom of what they can thrive in my personal experience. 

[ADMWNDSR83]

Thanks for that, @Cory.  The centerpiece of the tank is a pair of Koi angels, are they ill-suited for  the lower pH?  What is the best way in your opinion to bring that pH up?

[Ben Ellison]

Crushed coral I believe nis the recommended way to bring ph up.