Guppies and fire gourami dies suddenly

[Madipez2017]

So I’ve had this tank set up for probably two years and it started as a glofish tank, and I slowly moved it back to a regular multi species tank. 29 gal freshwater, ph is at 8.0, no nitrite, no ammonia, nitrate is at 5ppm (at last water change) temperature is a steady 78F/26C, good filter, air stone, while 9 yards......but suddenly (within past 3 days) I had my fire gourami and 3 of my guppies die. I can’t for the life of me figure out why. They didn’t show any signs of sickness and all my other fish are fine. I don’t believe my tank is overstocked as I have a 29 gal with 6 fish currently. I have 1 neon dwarf gourami, 1 glofish(skirt tetra style), 3 guppies, and a molly. I have live plants and drift wood as well as synthetic decor. All my fish have gotten along very well and the guppies only bother the skirt tetra really, he chases them off and then everybody is fine. I’m at a loss to understand why I had 4 fish die so suddenly. Any ideas??

 

[Demeter32]

Did the ones that died look odd at all? Are any of the living fish behaving oddly?

One theory is the molly. I know that mollies can be terrible fin nippers when they want to be and so can white skirt tetras. I'd watch the tank for a good half hour or more and observe their behaviors. I wouldn't put it past either the molly or the tetra to kill the other fish. A lone tetra often times becomes aggressive and nippy w/o a proper school.

Another theory is something somehow ended up in the water and killed the fish. If you haven't already, then I would do a water change just in case. Can you also test the water again and post the results before the water change?

 

[Madipez2017]

So I watched my fish for a while and the skirt tetra gets a tad bossy but it’s not picking or nippy, mainly just chasing my neon. My neon will usually go find his rock and chill out. He chases the others out. For the most part my fish didn’t give much indication that they were sick till it was too late. However my fire gourami was SUPER skinny! I tried separating him in my 5 gal quarantine, but he didn’t get any better. He just stayed the same, then died. The rest would lay sideways on the bottom of the tank and breathe rapidly. Then die in a few hours. My fire was the exception.

Now I did find out that my husband...(trying to be helpful) was feeding them extra. I only feed in the morning. He was feeding them lunch and dinner too.

PH 7.5
0 nitrite
20 nitrate (before)
5 nitrate (after)
However I did buy another test kit with a hardness tester. It was at like 1000! I knew the water where I live is super bad but is there anything I can get to soften it?