Fish dying suddenly with no symptoms

Tank
What is the water volume of the tank? 75 gallon
How long has the tank been running? 2 years
Does it have a filter? Yes 525gph canister
Does it have a heater? Yes
What is the water temperature? 79 degrees
What is the entire stocking of this tank? (Please list all fish and inverts.)
3 snails
4 bolivian rams, 2 dwarf gouramis, 1 bristle nosed pleco, 1 molly, 1 tetra, 1 angelfish, 1 upside down catfish, 1 dying killifish, 1 zebra loach

Maintenance
How often do you change the water? Every other week
How much of the water do you change? 35%
What do you use to treat your water? Api prime
Do you vacuum the substrate or just the water? Vacuum and change

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? Master kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia:0
Nitrite:0
Nitrate:20
pH:6.8-7

Feeding
How often do you feed your fish? Every other day
How much do you feed your fish? Brine shrimp, flake food, algae wafers, seaweed-all on rotatio
What brand of food do you feed your fish?
Do you feed frozen?. Yes Brine shrimp
Do you feed freeze-dried foods?no

Illness & Symptoms
How long have you had this fish?newest added Friday, currently ill over 1 year
How long ago did you first notice these symptoms?fridag
In a few words, can you explain the symptoms?listed below
Have you started any treatment for the illness?no
Was your fish physically ill or injured upon purchase?no
How has its behavior and appearance changed, if at all?

Explain your emergency situation in detail.
I added 2 angel fish Friday. Didn't quarantine (I know, I know). They were both very young and I knew it was a good chance we would have issues.

One dyed within 24 hours. No outward signs of illness/disease. Just swimming one minute, dead the next. I attributed to stress as they just came into LFS that same day.

Monday one of my oldest mollies died. Really didn't attribute to the angel fish at all as this is one from a multi generation inbreeding bunch I had. She was also the only female in the tank and constantly harassed. I have a 36 gallon just finished cycling I was moving the male to.

Tonight my killifish randomly bloated and doesn't seem like he will make it. He ate this morning, has been acting perfectly normal. Just suddenly swelled up and struggling to swim. I've had him for well over a year.

Other fish in tank: 2 dwarf gouramis, bristle nosed pleco, one molly, one tetra, one upside down catfish, one loach, 4 bolivian rams.

No other changes to tank other than I used a new bag of seaweed yesterday and some new plants last Thursday.

All other fish acting normal. No growths, eating fine, no erratic behavior.

Where do I go from here? I am really attached to these guys as most of them have been with me since day one.

 

When mystery fish deaths occur a good first step is to make a 50% water change & keep watch for symptoms of disease.
If your Zebra Loach remains healthy, chances are the situation isn't going to be all that bad.
Loaches are often the first fish that succumb to disease or bad water conditions.
Hope things get better soon.

 

When mystery fish deaths occur a good first step is to make a 50% water change & keep watch for symptoms of disease.

Agree

 

When mystery fish deaths occur a good first step is to make a 50% water change & keep watch for symptoms of disease.
If your Zebra Loach remains healthy, chances are the situation isn't going to be all that bad.
Loaches are often the first fish that succumb to disease or bad water conditions.
Hope things get better soon.

I did a 50% water change. Sadly I have now lost 3 additional fish from this tank (not the loach yet)and another fish in my 36 gallon.
Only common factors between the 2 tanks:
1. new fish added from LFS (different species)
2. New plants added Fri. Aquariumplantfactory.com
3. Started liquid C02 booster.
The cory that died in the 36 gallon had what looks like a ruptured stomach.
I have started API general cure and Melafix as that is all I could get locally with the assumption there is a parasite or bacterial infection at play.
Currently my 2 gouramis are struggling and doubt they will make it through the night.

Agree

Thanks. 50% water change competed and sadly the situation has worsened. I've had this tank running for close to 3 years with no issues. I've now lost more than half of my fish.

 

I'm not the go to guy regarding C02
information, but I do believe too much C02 injected into an aquarium can be toxic to fish.
Maybe one of our plant gurus can provide info about the toxic levels.