I have a planted 20 gallon that originally had six male guppies (in hindsight this was a mistake; they bicker often), nine fales julii corydoras, a nerite snail, and many bladder snails. The tank developed a green water problem which made it near impossible to observe the fish, so I got a UV sterilizer and it cleared up. I continued to run the UV sterilizer when one cory, who was the smallest and never really thrived in the tank, developed a swim bladder issue and continually slept on his side. Eventually he passed away from what I assume was starvation, as he had a dimple under his belly. He had no other visible symptoms. A few days before, however, a guppy died suddenly. There were no visible symptoms and he was acting normally the day before he died. He was also the smallest. I wrote off these two deaths as 'runt' deaths (and possibly old age for the guppy, as I've had him for a few months). The day he died I put the UV sterilizer on a timer as I no longer needed it to run 24 hours with the green water gone.
A couple days after the first cory finally died, a second cory got sick. She developed a bit of extra mucus on her coat, and would keep her fins clamped. The day before she died she would shimmer, but didn't have swim bladder issues. The day she died (yesterday) she began turning over and her dorsal fin deteriorated quickly. She also had some very small red blotches on her front, and the characteristic stomach dimple from lack of eating.
The day before the second cory died, another cory developed a 'corkscrew' tick, but after I changed the water (25%) she was fine. The day after, a completely different male cory appears to have a swim bladder issue, again sleeping on its side. As before, there are no visible signs of illness. He doesn't even breathe rapidly. I'm at a loss at what the problem could be, but since this is likely going to be the fourth death of the tank, I need to try to figure this out.
Right now I have Ich-X and ParaCleanse. I ordered Maracyn 1 and 2 but it will take a while to get. Yesterday I treated the tank with ParaCleanse, as parasites are my prime suspect. I assume that the red blotches were a secondary bacterial infection due to stress from the parasites or injury. Right now, the cory that developed the swim bladder issue is alive but on its side and will likely die. The other cories are resting. They don't seem stressed, but they also don't seem themselves. They have energy, as when they do decide to move, they move quickly, and do their normal foraging thing. My assumption is that they are bothered or cautious about the medicine that was added to the tank, and perhaps because of the new flow from an air stone I added when I started the ParaCleanse. I also didn't feed them today.
The UV sterilizer is off (but still in the tank) so as to not interfere with the ParaCleanse.
I never did quarantine tanks before, but I think I will be doing that going forward...
Water parameters (taken this morning):
20 gallon
pH - 7.6+ (API pH test, I ordered the High Range pH test, but it could be as high as 8.0 according to my tap water reports)
Nitrates - (0, floaters keep it at 0 constantly)
Hardness - ~300 ppm (according to test strip, but I orderd the GH/KH API test for better test; tap water reads 0 GH so I use wonder shells to raise this)
Nitrite - 0
Ammonia - 0
KH/Buffer - ~40 ppm (test strip, tap water is very high KH. Something caused the KH to drop quickly since the green water went away)
Water Temperature - 75
Sand substrate
HOB filter + airstone
I recently read that cories don't like high pH water. I didn't know the water was so high in pH until I used the API test kit (never use Amazon off-brand test strips!), but I also read that they don't like high GH water. There is a lot of contradicting information about this. If the wonder shell is bothering them, I'm wondering if aragonite or crushed coral will be able to provide calcium without as strongly affecting GH as the wonder shell.
Summary:
Corydoras died from swim bladder and starvation, a second is dying this way
A different corydoras died after shimmying, red blotches, and fin rot.
A guppy died for seemingly no reason
Guppies are acting normal; cories are cautious, but retain energy