Mass Wipeout

[Fifi]

I fed my fish as usual last night and as I went to put lights out I noticed some dead fish, 6. In the tank (4ft) are various size mollies all females, coz as soon as I see any males they go into another tank. As I removed the dead more were dying infront of me. Lost about 20 fish in total. I've tested the water parameters and all is fine, Not added any new fish to the tank, only had the additions of babies as they were born, no new ornaments or plants added, and can't see any symptoms at all on them and not even gasping for breathe, they are just dying. The dead fish are all ages from old mollies to various age babies and new fry.
The heater and filter are working fine, I have had this tank up and running for years doing regular water changes every two weeks, I have now moved the remaning fish into the hospital tank. I'm gutted, I am puzzled to what has happened.

Does anyone have any ideas as to why they have died and so suddenly?

 

[alexdallimore]

Carbon in filter? When last changed if used?

Failed Heater thermostat
(cooking them or freezing them)

Mass christmas suicide?

Treated new water correctly?

Children in the House adding things?

 

[cane76]

its a long shot but how about bad food maybe?

 

[WILDER]

What have you been feeding the fish as yes they can get food poisoning.
If there no gasping dosn't sound like a chemical has got into the tank.
Any signs of the fish flicking and rubbing against objects.
Are the fish laying to one side at the bottom of the tank.

 

[Tolak]

What's your water source, and what have the weather conditions been like in the last few weeks? This always screws people here, the water company jacks up the additives after foul weather, it's something to be careful of.