Red phantom tetras dying one by one

Tank
What is the water volume of the tank? 36 gallons, probably less considering decor and substrate

How long has the tank been running? 4 month cycle, around a month with fish

Does it have a filter? Yes, a tetra whisper X (or ex?) 45-60gal with one cartridge that came with it (never replaced, just gently rinsed once thus far) and filter bio media in the space around it, and a sponge on the intake

Does it have a heater? Yes

What is the water temperature? 77*, confirmed by analogue thermometer on opposite side of tank

What is the entire stocking of this tank? (Please list all fish and inverts.) 9x orange neocaridina shrimp, 6x glowlight tetra, 10x red phantom tetras (was 13), 6x albino Cory cats, 7x kuhli loaches, 1x mystery snail

Maintenance
How often do you change the water? 1x a week, or more often if water test indicates it needs changing (test everyday, or every other day, I'm paranoid)

How much of the water do you change? 20% give or take

What do you use to treat your water? Aquasafe into the water change bucket

Do you vacuum the substrate or just the water? Gravel vac (~30% of tank) and sand "cyclones" to get surface debris off sand (~70% of tank) every water change

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes, for four months using liquid ammonia until cycle was achieved, and algae started growing, and water tests showed the progression necessary, until 0ppm for everything after the last big water change

What do you use to test the water? Liquid drops tests from api for ammonia, NO2, and NO3. Litimus for hardness, alkilinity, and Ph
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0ppm
N02: 0ppm
N03: 17ppm (planning a water change tomorrow
pH: dip test says between 6.8 and 7.2

Feeding
How often do you feed your fish? Once a day, with a fast day a week

How much do you feed your fish? Enough for the tetras to finish in 2 minutes, plus an equal amount sunk to the bottom for the bottom feeders
What brand of food do you feed your fish? XTreme aquatic foods krill flakes, hikari freeze dried brine shrimp, bug bites tropical sinking formula for small fish, and hikari sinking wafers for bottom feeders small bites. It varies. The get the flakes every day, and one of the other things.

Do you feed frozen? No, terrified of parasites or disease

Do you feed freeze-dried foods? No, terrified of parasites or disease

Illness & Symptoms
How long have you had this fish? Three ish weeks???

How long ago did you first notice these symptoms? First fish death happened eight days ago, one yesterday afternoon, one tonight

In a few words, can you explain the symptoms? No symptoms, just dead

Have you started any treatment for the illness? No signs of illness

Was your fish physically ill or injured upon purchase? No

How has its behavior and appearance changed, if at all? Not at all

Explain your emergency situation in detail.
Eight days ago, I found one of the red phantom tetras drifting along the bottom of the tank. Still looked alive, beautiful color, eyes hadn't even fogged over, but wasn't moving. Not even gill movement. Scooped it up and plopped him in a bowl of room temp bottled Betta water thinking maybe he was just stunned? No change. Didn't even start turning white, or eyes clouding over, nothing. Beautiful little dude, except totally immobile, laying on his side at the bottom of the bowl with no gill movement. We had a fish burial.
Yesterday afternoon, exact same thing. And I mean EXACTLY. Since the first death, I have been closely watching these fish. No odd behavior, all the tetras (glowlights and red phantoms) shoal together, one of the glowlights looks a little far, but beyond that absolutely no hint of disease or injury or stressful conditions. They'll kinda spread out and do their own thing, then lazily group back together for a lap around the back of the tank, then spread out again. No tight schooling (except when the shouty 3 year old is in the room, maybe 2x a day) no erratic swimming or fast breathing, just beautiful harmony that is a pleasure to watch. But then the second one died, the one yesterday afternoon, same as the first. Still looked alive and everything, not being picked at by his friends or the other tank inhabitants, but dead all the same. The third red phantom (only red phantoms, no glowlight deaths) was found about 30 minutes ago, a little different. I assume he died the same as the other two, but his body got sucked onto the side of the filter intake, in the teeny gap where the sponge had slid down a bit. So he actually looked dead. White, cloudy eyes, the filter (I assume) had mangled his fins post mortem.

All the other inhabitants of the tank seem perfectly happy. The kuhli's are out and about (as long as the 3 year old isn't), the cories have chilled out and do their thing at the bottom (and occasionally swim up to the surface to mangle the floating frogbit), shrimp are all over and I saw one berried yesterday, and the tetras themselves seem healthy and happy (except that one fat glowlight)
I even found a cluster of unidentified white eggs on the side of one of the sunken ship decorations. So clearly all our fishy friends are happy. What gives with the red phantoms becoming actual phantoms, and how I can stop it from taking down the rest of them?

I'm sorry I didn't think to take pictures, but if another of these wierd "still looks alive" deaths occurs, I will take a photo

 

Sounds like you are doing a great job managing your tank.
Considering your Corydoras, Loaches & Shrimp are thriving, the water chemistry must be very good.
The Phantom's might be juiced, treated with hormones to achieve maximum color. That might explain why the Phantom's colors didn't fade after death. Although hormone infused fish often have shortened life spans, that's probably not the cause of the sudden deaths.
A strong possibility is that the Tetras were exposed to preventative meds either in transit or in the vendor's tank, which caused internal damage, leading to delayed sudden death.
Certain meds, especially Copper Sulfate, can eliminate bad bugs, but damage the guts.
Without symptoms it's very difficult to recommend any treatment, other than buy from a different vendor.
Wish I could be more helpful. Hope things get better soon.

 

Thank you so much!
My experience with fish in past was not the best (I was uneducated and uninformed and my fish suffered, although I didn't know it until this time around, over ten years later. I wanted to make sure that as I introduced my animal loving older son (7 y/o) to the hobby, we did it as right as we could. We're trying really hard, so these deaths have been really disheartening.

Would things like hormones or preventative treatments take this long to show? Since we've had them about three weeks at this point, I thought we were in the clear from issues stemming from the fish store. If that's the case, it really bites because all our fish are from the same place: the only LFS in town that isn't a Petco. However *their* suppliers...I have no idea. And I don't know if the store quarantines for any period before stocking in the sales tanks.

Would you recommend we take the risk at Petco then, or would you think an online retailer would be a safer bet to avoid things like this in the future?

Thank you for your wisdom and your time!

 

Fish are sometimes infused with hormones by unscrupulous breeders. I've bought obviously juiced African Cichlids in the past, only to have them die suddenly, w/o symptoms within the year of purchase.
I've bought plenty of beautiful healthy fish from Petco. They have a very good return policy.
Make sure any new fish you plan to buy are eating & hand pick each fish you want. Familiarize yourself with the symptoms of common fish diseases & be able to recognize the Ick or Oodinium parasite.
Petco as well as other big box stores buy their stock from big Florida commercial hatcheries.

 

For what it is worth, I never have had any problems with fish from Petco.