New tank, three deaths. Any theories?

[Jonatheber]

I stocked my brand 46 gallon new tank after getting the cycle up and running on Thursday. The tank is a mix of about 30 various guppies (6), platys (2), mollies (3), tetras (9, used to be 11- more below) and two catfish. In the three days with the tank, I lost a cardinal tetra, another kind of tetra (not a black widow, but similar looking and I forget the exact name) and a platy. Two of the three were sucked up against the intake of the Fluval 407 filter. (That's really the question - see below). The platy hadn't been moving much for a day, and earlier it died.

I did a water test - no ammonia, no nitrites, nitrates are around 5 ppm and the pH is between 7.4 and 7.6.
The temperature is 77.4 degrees.
The rest of the tank looks fine (including the plants), but last night so did the 2nd tetra that just died.

My question is, is there any possibility that the filter is too strong for the fish, and it sucked the two up against the opening and they couldn't get off while they were "sleeping" or something? I don't see any signs of disease in the tank at all, and the chemistry seems fine as well. I do note that another of the platys is hanging out in the corner near the heater lots of the time and isn't moving much at all, so I'm a bit worried for that one as well. Or is it more likely that a few of the fish didn't make the transition from the store well, and just decided they'd had enough?

 

[Brookster123]

Bad stock. It happens. Sometimes fish just die.

 

[kaphillips438]

Did you get them from an lfs or a big company store, places like petsmart barely ever do water testing or anything like that and thats why most of them die, but itcould just be that those three fish were already on their way out

 

[Jonatheber]

Do I have a killer guppy????

Did you get them from an lfs or a big company store, places like petsmart barely ever do water testing or anything like that and thats why most of them die, but itcould just be that those three fish were already on their way out

Response and an update...

I went to the largest single aquarium store in the Philly area. It is huge, and entirely (with the exception of a few reptiles) focused on fish. The place is pristine.

Update -
Another fish has gone missing. The most recent one seems to be one of the two male guppies I have vanished (leaving me with four females and one male). The rest of the tank consists of platys, mollies and tetras (e.g. painted glass, neons). Today one of the remaining platys was hiding in the corner, and seemed to have a chunk missing from the tail. I went to the huge LFS, and they took one look at the fish and said that something was picking on it. I have been watching the tank closely the last few hours, and the only oddness I saw was some of the guppies seemed to be pretty aggressive with each other (male being chased by the females, female chasing after each other, etc). And then I saw the single male trying to take a nip (a few times) out of the side of the platy that was hiding.

Is it possible that the male guppy was the problem? I thought guppies were pretty docile, and was more paying attention to the bigger tetras or the dalmatian mollies, who all basically seem to be keeping away from all the other fish.

Does the male have to go? Or is there some other likely cause and it was just a coincidence that the platy was being run at by the guppy?

 

[Autumnsky]

Check your water parameters. Often aggressive behavior pops up when the water parameters are unsafe.