Longtime lurker, so of course my first post is with a problem.
Tank is a 29 gallon planted with a female koi betta, 13 rummynose tetras, one (remaining) black neon tetra, two (surviving) Julii cories, and red cherry shrimp. I am running an airstone.
pH = 7.6
Nitrates = 25
Hardness = 300ish
Nitrite = 0
Ammonia = 0
KH/Buffer = 120
Water Temperature = 78 and trying to bring down
In the past day I have suddenly lost a rummynose, then a black neon I have had for about 5 years, and then another rummynose. Their bodies show some patches and bloating (see photos).
The remaining black neon has red gills (see photo) and a couple other rummynose have red patches. They seem to be having some trouble breathing and are not swimming with the school.
Yesterday I did a 10% water change and fertilized. I may have overdosed with iron since I used Easy Green and then added another squirt of liquid iron (Ultum Nature Systems) to keep the scarlet temple plants happy.
Today I've done a 20% water change, lowered the heater temp, and turned down the lights to "cloudy day" setting to try to reduce stress.
What might be going on and what are recommended next steps?
Backstory: One June 15 I brought home 6 Julii corys from my LFS. One died almost immediately and another two in the next two days. They seemed to have trouble swimming and kept sitting in the plants at the top of the tank. I lost another one on the 22nd. The remaining two seem to be doing fine.
Thanks in advance for any help!