New Fish Last Night, Old Fish Died. Unsure As To Why?

[emilythestrange]

Right, I had a 27 litre cycled tank for my female molly so she was seperate from the males, I brought a cory for her. They both were in the tank for at least 3/4/5 months ish. The female molly has had 3 batches of fry.

I changed the substrate and the tank, I got a 42 litre tank and put sand in it, de chlorinated it kept it up to temp, and let it settle over night and a few more hours until the water was clear.
I put my old filter in and the fish, A couple of days later i brought another corydora and a smaller female molly (last night)

My original molly whom ive had for almost a year was found dead about an hour ish ago. Ive got no clue as to why.
Worried, I checked my ammonia & nitrite stats, both are 0ppm (liquid API)

 

[emilythestrange]

Anyone?

 

[Sofia]

Did you put any old tank water in the new tank?
Did you just stick the molly in the tank at the same time as the old media?
Did you make sure the molly was slowly acclimatised before putting er in the new tank?

 

[Faythee]

That is odd. Did the Molly that died have any wound marks ?

I had a mean Molly before that tried to kill some of my other fish.

 

[emilythestrange]

That is odd. Did the Molly that died have any wound marks ?

I had a mean Molly before that tried to kill some of my other fish.

None at all, thats whats confusing me, it was a shock death.

Did you put any old tank water in the new tank?
Did you just stick the molly in the tank at the same time as the old media?
Did you make sure the molly was slowly acclimatised before putting er in the new tank?

Yes i put all of the old tank water in then fresh de chlorinated water and mied, then i put in the appliances and acclimatised the molly.