Sorry for hopping in this late and condolences on losing a dear Molly... But dropsy in fish or I guess more to say fish being susceptible to developing dropsy could be something genetic such as a bad immune system or virus infection, liver damage by internal parasites or bacterial infection.
In most cases, once it shows it already has damaged organs that regulate the fluid retention and it's about incurable and the fish is too far gone and too weak to recover anyway. Quarantine is always a must but treating it successfully is very rare and since the diagnosis why it developed dropsy is about impossible to determine from the outside if it is symptomatic of an underlying disease.
When you do your regular water changes and good husbandry anyway and all other fish look healthy and fit you can rule out bad water quality and parasites etc. Then it most likely is of genetic causes and a fish with an unfortunate immune system and or bad working internal organs.
Never a nice thing to say and always sounds harsh but euthanizing it is the most fish-worthy end to give it. Give it a nice warm bath with some drops of clove oil and it will slowly fall asleep within less than an hour and never wake up. 😢IMHO a better death than gambling over longer terms with all kinds of medication.
If parasite or bacterial infection is the cause then these symptoms would actually show long before dropsy develops. And your Cory missing an eye could be such a symptom... I also once bought a group of Cory's and noticed at home that 1 was missing an eye... After a while, it got worse and developed fungus on its skin and started flashing and died. Flashing is scratching with its body against surfaces, rocks and or wood.
I took a skin sample from the dead fish and looked at it under a microscope and found a "Trichodina" parasite. In larger fish this parasite likes to live in the fish gills and stay undetected, on smaller fish the can live all over the body and cause skin problems, if it ends up in the eye it will lose this eye. If untreated it will cause dead... Trichodina is a very common parasite usually latently present and healthy larger fish can take a few without showing problems. For tiny fish, this parasite seems to be a devastating monster... The only way to find out is a skin sample and a microscope.
Anyway... Infected fish that are more free swimming can also jump instead to try to get rid of an itch. Do your fish flash? Maybe you interpret this as active playful behaviour? Corry sometimes likes to gulp air, this also looks like jumping or could end up in an actual jump if it has an itch...
So Corry, missing eye, then missing fish? Flashing Jumping?
Fortunately, treatment is very easy and effective with Praziquantel... This is for the rest also a very mild medication with no negative side effects. Thus you can take a gamble and just give it a try... Then buy a bag of fluke solve read the description and add the recommended dose, the description says the treatment should be 24 hours only and do a water change. But this is for Flukes only...
Fluke-Solve® is the result of in-depth research by veterinary fish experts who have recognised the need for ornamental fish keepers to have access to high quality veterinary pharmaceutical products which are both safe and effective. The active ingredient in Fluke-Solve® is praziquantel which has...
www.fish-treatment.co.uk
In the case of Trichodina infection, you need to leave the same dose in the tank for 14 days without a water change. After 14 days change the water...
Then Fluke Solve becomes Trichodina Solve...
