Guppies Dieing - Need Advice

[NewBea]

I had my tank up and running for a good few months now - it was cycled, added all fish we wanted and water parameters settled and were stable for about a months now. (100 litre tank with livebearers only)
About 10 days ago I spotted one guppy missing which I eventually found dead 2 days later. I tested water - which was fine as usual and done the usual water changes etc. I thought I've done everything I can - I couldn't really explain the death so I just moved on.
But today I found another guppy dead. What's going on? Ammonia 0, nitrite 0 and nitrate around 5-10. I check and watch the fish every day and they seem happy, lively and fine - no chasing/fighting taking place. I don't see anything wrong with their fins or any other part of their body.
I literally have no idea why 2 fish had died and why the guppies in particular.
Any tips?
oh and I still have 7 molly fries in there - which are just waiting to be taken to the store - so if any of the other fish caused the problem/killed the guppies why didn't they first go for the fries assuming that they would be the most vulnerable ones?

 

[Tuxyu3]

How old are they? Guppies have pretty short lifespans and they could just be dying off.

 

[NewBea]

How old are they? Guppies have pretty short lifespans and they could just be dying off.

That's what I thought when the first one died but second time around is a bit suspicious.
I got them from the fish store about 6 weeks ago and I don't know how long the store had them - they were in a massive tank with various other type of guppies. I assume they couldn't have been in the store for too long - they normally try to turn the stock over fairly quickly aren't they.

I carefully checked the remaining 2 guppies and the fins are intact and seem active and healthy. If it was some disease surely it would affect other fish as well, wouldn't it?