My background as a freshwater aquarist is rather limited. I kept one betta fish for over two years before he had to be euthanized due to cancer.
Afterwards I decided to try my hand in a community tank, so I didn't completely clean out tank, only changed 50% of the water, before adding the tetras and another betta.
Initially I had 6 fish in an established tank: 5 diamond tetras and 1 very mild mannered betta. They got along for almost two months.
The issue started to occur a few days ago after I went on vacation for 1 week. I had done the water and filter change the day before I left, and had asked my roommate to feed my fish for me. When I came back, a foul smelling brownish gunk developed around the feeding hole, and the diamond tetras were lethargic. I suspect overfeeding.
I immediately changed out 30% of the water and proceeded to change out 15% everyday for the past two days. Second time I change out the water, I noticed one of the diamond tetra developed a bloody eye. The next day he died. Today after I changed the water again I noticed another tetra developed a bloody eye, and is at the surface gasping for air. This time I went ahead and scooped him out, and put him under a mild anesthesia so I can take photos of the infected eye.
So far all the other fish seem to be ok, but since this case happened twice I suspect it is more than just a coincidence. I just want to know what causes this symptom.
Below are standard info to give some environmental context. Thank you for your attention.
1. Size of aquarium (# of gallons)
2. Is your aquarium setup freshwater or brackish water?
3. How long the aquarium has been set up?
4. What fish and how many are in the aquarium (species are important to know)
One betta splenden and four diamond tetras
5. Are there live plants in the aquarium?
6. What temperature is the tank water currently?
84 degrees F
7. What make/model filter are you using?
Biorb Life S30, undergravel
8. Are you using a CO2 unit?
9. Does your aquarium receive natural sunlight at any given part of the day?
10. When did you perform your last water exchange, and how much water was changed?
11. How often do you perform water changes?
12. How often and what foods do you feed your fish?
Tetramin, freeze dried brine shrimp and/or freeze dried bloodworms. Twice a day.
13. What type of lighting are you using and how long is it kept on?
Biorb LED lighting. 14hour extended daylight
15. What are your water parameters? Test your pH, ammonia, nitrite and nitrate.
Last time I tested the water using the strips, everything was nominal (pH was about 6.8, no nitrite, small amounts of nitrate. Ammonium strip indicated mildly stressed but it has been like that for weeks despite the religious weekly water changes). Just took out another water sample to take to the aquarium for another test.
Also, one teaspoon of aquarium salt per gallon.
16. What test kit are you using and is it liquid or test strips?
17. When was the last time you bought a fish and how did they behave while in the pet store tank?
Almost two months ago, fish were spunky and would swim up and down excitedly when it is feeding time.