2 Dead, Please help

I hope someone can help me out here,
a few weeks ago i got a mango pleco, 1.5 days later dead.
2 days ago I got a gold nugget,1.5 days later dead.
I have know idea whats up,heres what I can tell ya:
water temp. 79.8* F
ph 6.8
ammonia 0ppm
the tank is a 40 breeder,its been running for 9 months.
filters are a emp. 400 & fluval 3plus or something like that.
40% wc every sat.
tank mates are all doing great,(EBJD,3 rainbow,3 rasboras & a huge snail)
Any help would be great,here are some pics before death

Die Fledermaus;521866; said:

How were they acclimated?

What are your other readings? Nitrite and Nitrate

Is there aquarium salt added to your water?

Did they look normal aside from being dead? (no missing scales)

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thanks for the replys
unfortunatly thats all i can test for,
no salt
they both looked like they were alive till i touched them,
The gn's top fin was straight up

WyldFya;521879; said:

Were the stomachs sunken? how big were they? Nitrites and Nitrates need to be tested. What was their diet? Is there enough surface agitation?

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I checked there stomachs before i got them & after-not sunken
They were both small-1.5 - 2"
the lfs was feeding omega 1 stuff & frozen bloodworm
i think theres is enough surface movement

WyldFya;521894; said:

What color was the poop?

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I never saw any

Thanks for your help Wyldfya,
nether looked bloated at all,
I just dont get it.
Can you suggest a good test kit?