Fish dying 1 after another

[mmcintosh_xo]

I'm getting so frustrated ?
I've been doing between 40 and 50% water changes, using prime added to the new water prior to adding, AND lugging the water from my mother in laws as my water has nitrate from my taps
I tested this morning
I have 0ppm ammonia
0ppm nitrIte
7.4 ph
Between 20 and 40 ppm nitrAte

Today I noticed one fish laying on the bottom alive, and others have red gills.
I intiallt assumed ammonia but its testing zero. It's definitely yellow but the yellow is a different shade then the chart. Next colour up in green. I tested my tank water, house water, and mother in laws water. All came back the same yellow so I assume that's a zero.
I have a Fungus eliminator that clears up serious fungus and bacteria. I'm wondering if I should treat the tank with this because anything I find about red gills/no ammonia is saying bacteria

Thoughts ?

 

[mmcintosh_xo]

Here is a picture of one fish with the red around his gills

 

[mmcintosh_xo]

Another angle showing the red gills. I'm not sure if it's what you would consider red gills or something else. I am new to the fish world