This is long... the TLDR is parameters all in check, diet is varied and high quality, Serpae and Cories sick, they are my only S.A. fish other than my Bristlenose.
Listlessness followed by signs of infection then death, symptoms have included paleness, Popeye, slime coat issues, dropsy, rapid breathing then death. It moves slowly in my Serpae, Pygmy Cories seem to go quickly with frantic erratic swimming and then death.
Treatment has been more water changes, Stressguard, UV.
The long version....
I somewhat suspect NTD, or some kind of mycobacteria.
The only fish affected seem to be Amazonian. I tried treating the first two sick Serpae in a QT tank with Focus and Kanaplex but the stress caused death overnight vs weeks in the main tank.
At this point I'm doing my best to avoid further cross contamination by soaking tools in bleach water then prime and running scalding hot water through the hoses for several minutes between water changes. The two problem tanks are getting water changes with dedicated buckets and hoses.
Parameters are always 0,0, <20. PH is 7-8 depending on hardscape and CO2 in the tank. Every tank gets at least 50% water change weekly.
Tank 1 60 gallons lightly stocked semi aggressive/nippy fish. 4 Kribs, 3 SAE, 10 Leopard Danios, 1 female BNP, 5 (about to be 4 Serpae)
78°, healthy plants, UV running in canister, DIY co2 with citric acid and baking soda, drop checker is turquoise which is just enough to speed up plant growth.
The Serpae are dying off one at a time, they get listless, then show signs of infection and then die. They looked rough when my LFS got them in, but she held them for several weeks and quarantined and treated and they were much improved when I brought them home.
This has been slowly happening for months and at this point I'm thinking I need to cull them to avoid further contamination or spreading it to the other fish. I really hate the thought though.
No other deaths or illness in tankmates. The danios always have fin damage but it heals quickly and they are happy and very (VERY) active.
I've been doing twice weekly 50% changes and dosing Stressguard at full strength with no improvement in the most obviously sick Serpae. He's still eating and schooling but has peeling looking slime coat and a raised white lump that has been slowly getting bigger.
It isn't fuzzy or stringy like fungus or columnaris.
Tank 2 20 long, 8 Boraras Maculatus, lots of RCS, some pygmy Cories, 2 Oto.
76° PH 7.5, was heavily planted with lots of Mopani and Manzanita. DIY C02 with champagne yeast. Drop checker was bright green but I've backed it down to turquoise.
The Cories aren't out and about much and the dying ones start swimming erratically and breathing heavily then die within a few hours.
I had a small school of 6 Cories and they hid constantly, after months of waiting I was finally able to add 12 more. Again my LFS quarantined them, they were all healthy and active and I went ahead and added them without further quarantine, which I know is risky. I lost a couple in the days immediately after introduction and they were all still hiding a lot so I've been fairly hands off with the tank, only doing water changes and spot treating some BBA with excel.
The tank is heavily planted and has lots of driftwood, after losing 2 Cories last week I pulled the worst affected plants, stopped using Excel and I've dropped the water level to increase surface agitation.
I lost two more on Tuesday.
I started dosing Stressguard at half dose last week but stopped because my RCS looked pale. Added carbon and turned on the UV in their canister. At this point I can only find about 8 of the Cories.
(I just found another dead one while typing this out. The shrimp colony seems thinner too. I have no idea how many Cories I have at this point and I'm considering taking out more of the shrimp, plants, and driftwood and dosing.... something??)
29 gallons 15 CPD, 6 cheap endler hybrids, just had the first deaths last week since being set up in early September.
The CPD started spawning and I lost 1 male and my 2 biggest females. No signs of problems except for a few light patches that looked like minor injuries and then quickly death.
Head counts every morning, everyone is bright and hungry.
Tank 4, 20 gallons low tech, 2 dwarf puffers. They are fat and sassy. I'm no longer feeding snails picked out of other tanks and have gone to bloodworms and BBS, I just ordered a daphnia culture.
5.5 gallon low tech jungle with 3 new Sparkling Gourami. I had planned on going back for the rest and giving them the 20 while splitting up my puffers. At this point everyone is staying where they are.
Tank 6, 4 gallon Betta, he's lazy and a poor swimmer but very happy.
Tank 7, 55 with young 4 fancy goldfish, they are my sweet wet pets, I'm most concerned about contamination here, they get water changes first and have their own cleaning tools.
Possibly unrelated but last December I had a small owie on my hand that didn't want to heal as well as a rash on both hands and forearms a couple weeks later. I used peroxide several times followed up with Mupiciron ointment for 10 days and water proof Band-Aids. I actually thought I was allergic to prime but the rash lasted about 10 days. I use a squeeze pump to start siphons now and cover any cuts with water proof Band-Aids.