Goldfish tank infection
I hope I can get some advise.
I have a 250L tank with previously 5 goldfish (3 fancies, 1 big oranda, 1 butterfly) and 4 danios. Water parameters and stable tank for 3 years; partial water change weekly 30ish% scheam water conditioning. Sand bottom, very little decoration plants. Filtration 1x 104 canister fluval and 1x 406 + air pump.
We moved houses and I think the cycle crashed after this as I got an ammonia spike (unfortunately I could not reinstall the big filters straight away).
I did water changes treated with Scheam conditioner and tetra start cycle stuff and in 3 or 4 days ammonia disappeared.
That was like 2 or 3 months ago : since then I lost a fish due to swim bladder and dropsy every 2 or 3 weeks... started with the big oranda- he had been quieter for months but I though age (he was 6 years old) and size (600g). Noticed dropsy- treated in hospital tank with injectable antibiotics (I am vet but not fish specialist). Got worse- euthanised after 3 days.
Then 1 day after water change the butterfly got signs of dropsy and floating issues- same treatment in separate tank and he surprisingly got better (I used injected baytril and metronidazole in water). after 1 week I stopped treatment and he relapsed - euthanised a few days later .
All the other fish appear normal during this but I treated the tank with comercial treatment for bacteria containing formaldehyde. I had always had high nitrates in tap water but apart from this other parameters were fine.
2 weeks later or so the third one (a fancy) got swim bladder also after a water change. Euthanised today.
Other one has mild fin rot. I treated the tank with methylene blue this time (what I had atm).
I am not sure if I have an stabilised bacterial infection in the main tank... (as one fish got much better initially with antibiotics)... maybe can be my tap water???
Is it worth treating with Melafix the whole tank? Shall I use something else? Do I need to treat everything with antibiotics and restart cycles? I do not want to do this as daily water changes to do a fish in cycle now in a big tank is so difficult...
Thank you so much!!