Why Are My Fish Dying?

[westonsupermanc]

I like to think I am well read and experienced in fishkeeping however my middle tank of 3 has lost over 30 fish in the last 5 weeks or so and I do not know why. so I am looking for ideas out there people maybe something I have not thought of. The tank  is the cleanest of the 3 and is a 64litre tank full of young platies that I have bred from another tank also gobies mollies and a corydor.  please allow me to say what I have tried so far to eliminate lots of suggestions. Had the water tested and its fine. done lots of 50% water changes also washed all of the plastic plants and ornaments several times. Treated the water for fungal and other infections to no avail. Added more oxygen to the tank also have bought 3 separate utensils for each tank so as not to cross contaminate. Finally inspected each dead fish and there were no signs of injury or infection to any of them. The only exception was yesterday when a brand new platy was introduced he very quickly lost all of its colour and died within an hour. I quarantine all new fish and float them too so found this a little unusual. My other tanks a 128 litre and 48 litres are doing well so cannot think poisonous gases are responsible. Any ideas please as it hurts to lose so many fish especially as a lot of them I bred myself.

 

[DerpPH]

Hmm... how long was it running? maybe it was old tank syndrome

 

[Alm0stAwesome]

Hi there, my quick reaction is to think that you've got a lot of fish in that tank.
 
Do you have a water test kit? Without actual readings it will be difficult to help. A lot of people get told their test results are "fine" but we don't actually know what "fine" means. Have you done anything to the filter recently? Changed sponges or cleaned it?
 
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