Any ideas on what could be wrong with my tank? Fish death

[sonicboom]

I've been having problems with my tank recently, likely after introducing something despite quarantining new fish.

Symptoms:
Occasional rubbing/flashing - noticed this with rams and corydoras in particular
A few weeks later 1 younger corydoras started swimming continuously (day and night), hardly ever resting, often going to the surface to take a gulp of air and return to swimming. The corydoras within a few days developed swim bladder buoyancy of floating upwards, and become thin, and died.
Now I have another young corydoras swimming continuously, gulping air, and has some floating upwards issues too.
I do not see any outward symptoms now, though I previously saw on a few harlequin rasbora a small white growth on their mouth, and an otocinclus with a single white spot on a fin (symptoms gone).

Tank and Parameters:
Well stocked, fully planted 160 litre tank with a lot of hiding places and little fish aggression, and corydoras continue to breed
25 C / 77 F
pH 7.5
Ammonia 0 ppm
Nitrite 0 ppm
Nitrate 15 ppm
KH & GH within normal ranges for my fish
Regular water changes

Treatments so far:
Aquarium salt - low dose
Medication for White Spot, though I am not sure the diagnosis was correct - Malachite green
eSHa 2000 - Ethacridine lactate, Copper sulphate, Proflavine hemisulphate
Sterazin for Gill and body flukes - Acriflavine, Malachite green, Piperazine citrate
(Current treatment, along with salt) Bacterial medication - Formaldehyde, Bronopol, Benzalkonium chloride

I'm not seeing signs of velvet, white spot, gill flukes, other than flashing - the two corydoras that keep/kept swimming do not flash - all other corydoras are acting normally.

Any ideas?

Do I treat for parasites again, reduce salt concentration or something else. Other medications I can buy in the UK for the symptoms I see seems to pretty much contain the same ingredients as ones I've already used.

Any thoughts much appreciated! Thanks

 

[connorlindeman]

Wow thats a lot of meds. Not sure what the problem could be but its best to have positive id before treating.

 

[sonicboom]

Yes, it's been a lot of medications - I've tried over the months to identify what the problem has been, but still unsure. I've also noticed flashing with the harlequin rasbora, and one has lost a lot of its colour and is getting picked on.

 

[Goose3080]

Yes, it's been a lot of medications - I've tried over the months to identify what the problem has been, but still unsure. I've also noticed flashing with the harlequin rasbora, and one has lost a lot of its colour and is getting picked on.

Its common for fish to loose their colour if they are being bullied, if you see it happening, tap on the glass each time it happens, this will scare the bully away.

 

[Boundava]

Its common for fish to loose their colour if they are being bullied, if you see it happening, tap on the glass each time it happens, this will scare the bully away.

Don't tap the glass as that bugs all the fish. Either use a tiny net and chase the bully around a bit, or use the handle end of the net to chase the one bully. Most fish will stress at first but soon realize they aren't the target of your aggressions, lol.