My Poor Fish

[GetItSahn]

I made a post about this maybe 2 or 3 days ago. Just wanted to see if I could get some new feedback/opinions.
 
Basically I had two guppies, two mollies, 6 corys snails and shrimp. EVERYTHING was perfectly fine, the water quality, the plants, the fish everything!
I decided to add some more fish, I bought 4 more male guppies and a honey gourami. For the first three days after I got them, a guppy died everyday with a missing tail (it was deteriorated). I still have one guppy from that purchase who has survived. 
 
My Honey gourami's tail is getting blacker by the day. I feel bad about how it looks but my gourami is acting fine! Before the guppies died, they showed signs of being sick but the gourami has shown nothing.
 
Those 5 fish that I've had problems with all came from the same PetSmart, So two days ago I went to a different PetSmart and bought a Clown Pleco and 6 Neon Tetras. Everything is going fine so far with those newest fish.
 
Anyone know why the 5 fish that I bought were so troublesome? None of my original fish that they were housed with got sick or died while with the 5 "bad" fish. Could it be that the PetSmart takes bad care of the fish so I got them when they were already sick or super stressed?
 
I just find it hard to believe something is wrong with my tank if only the new fish from THAT PetSmart get sick.
 
Thanks for everyone's advice and for viewing this!

 

[ncguppy830]

Tank size?
water prameters?
Age of the tank, how long have you had it running since cycling?
newly cycled tanks can easily get buildups if you add too much fish, that is why its reccomended to get fish perodically, assuming this is the problem.

 

[GetItSahn]

It's a 29gallon.
 
All parameters are 0 except for nitrates which are 5
 
I'd say the tank tank cycled for 2 weeks about 2 months ago maybe 2 and 1/2.
 
I added those 5 fish 1 week after  buying my last batch. I wait 1 week to put in new fish whether its one fish or 5.
 
Hopefully that helps you!

 

[ncguppy830]

well, its very possible its petsmart fault, all of there tanks are connected and if one fish gets sick, they all do. They best thing you can do is not go back to the persmart.

 

[GetItSahn]

I've had problems with mollies from there in the past. But I received corys from there and they are fantastic. I'll do what you suggested. 

 

[rpgmomma8404]

I don't seem to have luck with guppies. I did some research and it said that even in good conditions they can die without any reason. I'm not sure how true that is though. I just be careful about what petsmart you go to. Here we have some really good petsmarts and petcos then we have some really crappy ones (like dead fish all over the place, cloudy tanks and such).