Hello. Sorry for missing this post this past week.
How are the fish?
Have you changed the media or rinsed them in tap water with out dechlorinating first?
It is apparent that at some point there was a trigger to cause issue.
Likely the heat certainly is not helping you.
There seems an insufficient amount of water changed previously and 20% per week is common. And any time there would be an issue such as what you have you could as you have tarted to increase the pwc
's and perhaps 2 times per week of the 30% might be a standard while it is hot.
Heat causes bacteria to thrive
Heat causes some fish to become stressed
The warmer the temp of the water the less O2
is available for the fish and adding an air stone would help as well or lowering the water line a couple inches / several cm to create more splashing /aeration. Which can cause more stress.
Stressed fish get sick easily.
Fan is a good start for some cooling!
One possible option (whether this is feasible???) If you have very hot weather conditions seasonally, worst heat over just several months you might consider using frozen water bottles to help cool the tank maybe adding in the morning when you leave, having a set for freezing and a set for using, and switching them when you get home.
Check for moving from hotter area by a window maybe to a lower cooler spot.... or other such possible helps.
Cotton mouth disease is a gram negative infection. Neomycin and acriflavine treats that but it might have developed resistance, or need a longer course of treatment like 10 days or longer. I can not say and not experienced enough, to say for sure. Just thinking of how taking antibiotics as a person can be.
The more usually recommended treatment medications for the disease are kanamycin and nitrofurazone. Maybe they are available?
As for the vac'ing you would just need to clean where you see mucky dirty water coming up.
Checking how much you are feeding might help as well.
Slow down feeding and just feed an amount they eat every minute, then add a little bit, make sure they eat it, and so on until the amount is fed. If you feed just once per day consider dividing their food into morning and evening portions.
This will help reduce waste which also causes the more mucky substrate. And reduce vac'ing need. Maybe just vac under once every few weeks. Maybe you can lift a decoration while vac'ing and then set back down, minimizing disturbance.
Foods with lots of fillers like grain meal will cause additional waste too. Most foods will contain some of these for the production of the shape of pellets wafers, etc.
Stressing them out with a big disruption of the whole tank does seem problematic.
Maybe focus feeding on one side of the tank with less decoration (might need to change / move some decor) and it will also help keep mess in an easier spot to vac.
Keep up water changes.
Also you might want to sanitize the nets and water changing things after each use until the fish are well. Bleach is what I usually use. Not sure on concentrations for products where you live.
"What percentage of bleach is in Clorox brand bleach?
At 7.4% sodium hypochlorite, Clorox® Disinfecting Bleach is more concentrated than most other bleach products." web
You can use 1/4 cup / 60ml to 3/4 cup / 177ml of bleach per 1 gallon / 3.785L to sanitize vac, nets, water holding containers. I soak them for up to a day.
Triple rinse very well, then add dechlorinator to soak for up to a day. Rinse again and air dry.