Yes they are the tanks I have, 125l is the effected tank 90l is clean, I do not use any of my nets/filters/gravel cleaners or any other tank equipment for both tanks, I have 3 filters in my 90l and 2 in my 125l, I also have 3 nets and 3 gravel cleaners.
I understand that in an overcrowded tank disease/illness can spread easier, healthy fish have better immune systems. As for water changes, I do at least a 70% water change on sundays and that is the same day I do not feed my fish. As I mentioned I have 3 gravel cleaners, 1 large one, 1 small one, and 1 that is battery powered this one has a hose attachment and a porous bag which you can attach and it sucks up all excess food and fish waste (poo), after my water change I spend the rest of the week cleaning out the tanks with the gc that has the bag so that there is nothing in the tank to decompose and effect ammonia. I have never had a big problem with my stats, they usually only increase by a tiny amount towards the end of the week.
The 90l only has a pair of kribs, a pair of BN plecos, a small zigzag eel and a red bellied newt. The 125l when it was stocked with all my fish before the losses was fed; 2 blocks of bloodworm, 1 block of brineshrimp, 6 small pleco wafers and ground up flake food for the shrimps, my kuhlis always filtered the sand during feeding so I put in extra flake so as they were filtering through the sand they would get food if they were unable to get any of the frozen food.
When I started treatment the carbon was removed, i never reuse carbon once it was removed I just binned it. When I used the whitespot meds I continued to filter the sand with the bag filter then once I had stopped I left it a day then done a 50% water change, I treated with fungus control for a couple of days then done an additional 50% water change, after that I had lost the majority of my fish and I removed 20l of water then treated with methylene blue. Have not done a water change since but will be doing a major clean up of the tank on friday. I think by that point all sick fish will be gone.
The cockatoos were for breeding but they were going to get there own tank, I only had them in the tank until they were slightly bigger and and I could cycle their tank and get the stats right for them before they were introduced.
I will be trading in my f bettas and red robin gouramis and the female cockatoo cichlids, possibly the hoplo if he recovers. Once they are away I will see if I am still able to get my bigger tank. I will be getting a 60l quarantine tank first so I will not have a repeat of this incident, I hope I will be able to bounce back and get everything running smoothly again.