Rainbows dying one at a time

[AJA1976]

Fairly new 29g tank. Heavily planted, properly cycled. pH 7, KH 80, CH <30, temp 77, NH3 0, NO3 0, NO2 0.5, Twice week small water changes. Slowly stocked with 5 danios, then corys, then 3 German rams, 1 female Krib. Recently replaced danios with neons and added 3 more golden rams. Lost a couple of neons, understandably. But then one blue ram began to slowly sicken and die. Zero visible problems other than color loss, rapid breathing and no eating, turning into swim issues, then death, all within a couple day or so. Then, one at a time, the other 2 blues experienced same issues and died. No two ever sick at once. Lost all three within a week. No obvious bullying, Now one golden ram found apart from others and rapid breathing. WTH is happening???
update- now 2 goldens presenting same. Cannot find any exterior symptoms

[Tech Support PE]

Hello AJA1976,

I'm sorry to hear about your fish dying, that's always saddening! I can think of two things from what you wrote that may be the cause: #1- Nitrites: if you've got nitrites it means your tank isn't fully cycled even though you've got zero ammonia or fish deaths have caused an acute ammonia spike that the biological filter is trying to adjust to. #2- One or more of the fish you added were sick and have infected others.

My recommendations are:
1- Dose Prime as directed on the label for the next week to make sure there's no free ammonia in the water that could be stressing or harming your fish.
2- Don't add any more fish until nothing's died for a month. If, in that time, once you've achieved zero ammonia and zero nitrite, you notice signs of illness, add PolyGuard as directed. Do not dose Prime while medicating.

I hope this helps!

Regards,
PE