I am in a very complicated situation here. Don't understand how to proceed so looking for a step by step guidance.
I have a 80 L planted tank with almost 19 fishes, mixed varieties and 3 mystery snails.
The filter is a mini canister filter with around 500 L/H capacity. Bio media is Seachem Matrix. Also I use filter flush, activated carbon and sponge inside it.
The water parameters are as follows
Temp: 26 C
pH : 7.0
Ammonia & Nitrite : 0 ppm
Nitrate : < 20 ppm
The problem is, there is some kind of fin rot bacteria there. When I add fish to the tank in 1-2 days many of the fish get affected by it and start to die. it happens to almost any kind of fish I add. The existing 19 fishes have immune system strong enough to survive and they are happy and healthy also growing in size.
The tank with the fishes are running nice for more than 3 months now but whenever I am adding new fish in 1-2 days they get fin rot problem and start to die. I saw few dead fish with almost no fins but most of the dead fish get eaten in the night by other fishes and snails, I find no sign of them. I added an otocinclus few days ago, He is alive moving around but the rear fins are kind of gone.
Regular water changes, cleaning everything in the tank even changing the full substrate did not help much. Only delayed the fin rot for few days.
I want to disinfect the whole tank so that I can add some guppies and more otocinclus etc.
I live in Brazil so we don't have all the things that you can get in US or anywhere. We have something called "Labcon Bactor" which contains neomycin. Here everyone uses it for curing the fish from fin rot. The indicated dose is for 7 days. But in my case I am worried that if I add it to the tank I will lose the beneficial bacteria in my filter and lose the nitrogen cycle too.
I don't understand how can I proceed so the tank can be disinfected and the cycle will not get affected. I will be very grateful if someone can advice me a step by step procedure.