Neon tetras dying

[Simmo]

I have a problem in my low tech, planted Juwel Lido 200litre community tank. Been established with quite light fish load for 18 months with no losses now neons are losing condition one by one, I had 16 now have 10 or 11. The sickly ones show one at a time, emaciated, fins twitching and I have been euthanising when they appear terminal but nothing else obvious? Fish are same age as the tank. No fin rot or bumps, bulges, lesions etc. Other fish appear unaffected (Honey gourami, corydoras) although I have lost (literally) my fairly new female apisto borelli and One of four honey gouramis n the last 3 months which is quite high mortality.
Water is as soft as it comes, temp 24, 50 litre w/c weekly. Zero ammonia, nitrite. Possibilities, treatments appreciated.
Sickly fish and a healthy one below
Thanks

 

[PARAGUAY]

Something in the tank,you say been established a while, must have changed have you fed any live food lately or put plants in? Have any dead fish been in for a length of time. It looks like Neon tetra disease and no cure and affects tetras in particular. Can you quarantine if not plenty of water changes

 

[PARAGUAY]

Forgot you asked, esha2000 is a med for many ailments and bacterial infections and Melafix is a all round treatment

 

[Simmo]

Thanks for the replies Paraguay, I suspected Neon Tetra disease, can it lie dormant, the last new neon went in at least a year ago. There were unrecovered corpses, the female Apisto, a honey gourami and maybe a neon or two. Just vanished as they do! Thanks for the recommendations, I’ll buy one of those products. Nothing else has changed except I reduced water changes to 25 l per week from 50l per week (from 25 to 12.5%) as an (unsuccessful) experiment to see if less water changes might stabilise CO2 a little and reduce BBA, back up to 50 l a week now, I’ll bin all the fish food and start afresh too.
Appreciate your input
Cheers

 

[fishkeeper1]

Hi what is the out come of this?
Did you get more Neons or a diffrent breed of fish, I think a freind has had the same issue with there Neon Tetra's i.e Neon Tetra Desise.
I was thinking if they leave the tank empty for a long period of time (6 months) this would allow what ever is causing the issue to maybe die off as the tank stats are all ok.
Thank you