They would seem to be a thing in your aquariums. You're speculating off a very tiny sample.
I suspect you need to look at what is going wrong in your set up. I would look at over medication to start - I have never needed antibiotics for a killie species, and you are using them with abandon. I would also look at diet, temperature and maintenance regimes, and if you decide to stay with killie keeping, eliminate each possibility methodically before jumping to conclusions.
In my larger sample, killies have been remarkably free of bacterial infections. The only major issues have been parasites - Camallanus once in 35 years, and ongoing issues with Oodinium parasites - easily treated if you spot them. The issues you're having with generally hardy little fish are quite remarkable.
I have only been adding antibiotics to treat fish with confirmed external signs of infection, and for ones that do not yet show external signs but are lethargic, I isolate them first (to prevent disease transmission).
So, for the clown killies and lampeyes I have tried a range of temperatures from 20~26 degrees C. I have found them to behave best at 24 degrees C, being most active without raising temperature too much. The clown killies seem to do fine at a slightly lower temperature as well, but there is more flashing below 24 degrees C.
For diet, I am feeding twice a day, each time with 1:1 ratio of flake and brine shrimp at 2% body weight (because bbs is mostly water, I tend to feed them more). In 1 out of every 3 feedings I grow out the bbs and enrich with fatty acid, amino acid, probiotics, and algae. I am not sure if this is necessary. There is on average 2 or 3 pieces of poop per fish per day.
I have been maintaining the tank with 30~40% water change per week, and have recently switched to a canister filter, making alterations to shower pipes to create very gentle water flow. I am planning to clean the filter every 3 months. Before, it was a hang in back filter with mat replaced every week and biological filter not cleaned at all (for about 6 months). During each water change, feces is vaccumed, and half of aquarium glass is wiped with sponge and water surface biofilm is scooped with a jug. I do not mess with the canister filter.
While there is not much ammonia or nitrate or nitrite (everything is below detection limit), plants appear to grow very fast, and there is not algae or worm, so I think the cycle is established fine.
Tank 1 has 14 annulatus in 56L, 10 are born from 4 parents (initially had 16, 2 dead)
Tank 2 has 6 annulatus in 80L, all born from 4 parents in tank 1 (no apparent disease history)
Tank 3 has 6 lampeye in 12L (no apparent disease history)
Tank 4 had 2 aphys in 7L (both dead; also tank had no plants only fake decorations)
Each tank has stable pH of 6.8 and GH 40ppm, just straight tap water with chlorine and heavy metals being treated
Sorry for typing out a novel with every single detail- I really want to find out what I am doing incorrectly, what's totally unnecessary etc. bc it's my first fish. To prevent contamination among the tanks, I use separate equipments and buckets to perform water changes, and water changes are always performed on different days. After performing a water change, I immediately sterilize equipments and take shower myself. After handling each tank, I spray relevant areas with isopropanol, and each time I touch anything that goes to the tank, I wash my hand with soap and rinse thoroughly. When I introduce a new fish to a new tank, I assume that the water is full of disease. I have equipments dedicated to new introductions and wear gloves such that my hands will never come in contact with the water in such tank. Sterilizations are done with 10% bleach 1 hr followed by isopropanol spray/wipe followed by water rinse, with equipments sure to contact with disease a 2nd bleach sterilization is done and porous objects being trashed
I would be curious to find out which of these steps might be unnecessary, or if I need to be doing even more, etc.
Also, it may be helpful to mention that 6/10 fish that died were anorexic and remained anorexic literally minutes after being introduced to the tank (no chemical being present), 2/10 fish became anorexic within days of being housed with one of these 6 fish. For the other 2/10 sick fish (both clown killies), 1 became anorexic after being bullied and 1 became anorexic for no apparent reason. The first sign of disease has always been anorexia, and in 9/10 of these cases, no drugs were present in the tank when fish started to exhibit anorexia.
A bit more detail on the above 2 clown killies that died were,
1- Bullied and then turned anorexic at 3 months old, exhibited a lot of flashing and darting, dashing, cured by formalin. Formalin could not cure the anorexia and died within a week in hospital quarantine (December)
2- Turned anorexic for no reason at 3 months old, exhibit no flashing and survived for 1 month in high dose kanamycin, at late stages showed darting and jerking movement cured with formalin. However formalin could not cure anorexia and died soon afterwards with a melted jaw (February)
I think because we all have different experiences with different tank set-ups, so I want to also hear if others have suggestions as to what I should change, not change etc.
As for speculations- when I speculate that killifish may be susceptible to incurable bacterial infection, it is not based on my experience (because I have only obtained 3 species from 3 sources). Rather, Dr. B killifish section of disease book mentions that killifish are susceptible to bacterial infection (I bought this book, because I am a fan of his videos and I think his diagnosis is very reliable), and also based on what is written on the website of killifish association of my country of residence. When I speak to different people their experiences are sometimes conflicting. Because I think very little actual evidence is present, with most being anecdotal, many different speculations can be possible.
As a side note, I completely agree that killifish are tough fish. I have seen a clown killie recover (apparently completely) from wasting disease and being moribund for weeks. They have very strong immune system. Many killie "experts" whom I have spoken (I have been doing a great deal of research after getting the aphys) mentioned to me that wild killies are extremely difficult to keep in captivity and cannot recover once they experienced bacterial infection due to change in water parameter. (Just because I have spoken to them of course does not mean I believe them)
3/17 Update: I got a new pair of wild A. cognatum from a different store, this one being a fish store that sells expensive fish. It arrived in warm water and I was being super sloppy acclimating them (on purpose) and was pretty much doing everything that the store owner told me not to do. These feed just fine. While they arrived with anchor worms, THAT can be dealt with, and I'm just so happy that for once they didn't arrive with random mouth bumps
I think I'm going to have to do everything wearing gloves now so as not to contaminate, though I don't really know if that's necessary since I don't really have much experience.
