Urgent Now!!corydoras Fin Loss Loss Of Balance Pale

The problem is knowing what to treat the fish with. If you were in the US it would be obvious - an antibiotic. But we can't get antibiotics without a vet's prescrition in the UK. Some of the symptoms suggest an external bacteria problem, some an internal.
Of the meds you list, melafix and pimafix are the least useful. Melafix is more of an antiseptic than an antibiotic (think of it as dettol for fish, good for stopping infections getting in but not curing them once they have a hold) and I personally have not found pimafix to be useful, though it does say it treats internal and external bacterial infections.
One of the better treatments is myxazin by Waterlife if you can find a shop that stocks that brand. Or possibly eSHa 2000.

Since you already have it you could try the pimafix, though it has not been successful for me.  I have used both myxazin and 2000 for finrot and both did work eventually. Whether they would help your cories, I don't know.




Grasping at straws, you said you haven't tested your nitrate yet. I know that some fish are adversely affected by high nitrate, though I didn't think cories were among them.
What is your substrate like? Unless you have changed it to something sharp recently, it is unlikely to be this as you've had them trouble free for so long.
Do you regularly clean the gravel/sand on the bottom of the tank? Build up of debris there affects bottom dwellers more severely.

So long as the gravel is smooth, there shouldn't be a problem. It's gravel with sharp corners that can cut cories' mouths. Since the problem has only just started, it would only be the substrate if you'd recently changed to sharp gravel. A long shot but I needed to check.

Do you have any other fish (eg loaches) that could be eating the snails? If not, why are they dying? Is something killing them that could be affecting the cories? You haven't added anyhting to kill snails have you? Dead rotting snails could in theory affect the cories as they root round in the gravel after bits of food.

sorry its taken so long - ok the Nitrates Ive tested are between 10ml and 20ml on the colour chart...the colour shown in the test tube was bang in the middle of the 5 colours on the chart. The chart goes up to 40ml and my tank shoes between 10ml and 20ml...probably more like 15ml of nitrates.  I know this isnt good, but it has been the highest on the colour chart before now.
Ive read that real plants can lower the levels of nitrate, the snails are currently munching their way through the plants.. i dont have a heavily planted tank.

Last night I had to euthanize the fish that was ill....it was horrible. I got home from work and the tail had completely corroded..just jagged and red...and the fish looked pale, poor thing:( It was on its back on the floor still breathing but couldnt swim or move...

Ive also tested the ph level of my tap water and its between 6.5 and 7 (same as the tank shows).

I am at the point now of putting in Myxazin...i havent done it yet, but thinking tonight...??