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Well it seems like you're in a situation that is (unfortunately) one of the most common: The formal term is I'm-doing-everything-right-and-none-of-the-symptoms-are-very-diagnostic-wtf-is-going-on.

TL;DR - Get a UV filter, run it for a week before the new fish arrive, once you start to see babies wait two weeks and then remove the UV filter.


Long answer:

The cory death isn't too helpful (in terms of providing information) simply because about 10% of fish from the pet store seem to die no matter what anyone does. In fact, your survival rates are pretty good a year after the purchase date. You tank set-up looks about as good as it could possibly be, so I can't imagine the environment is unusually stressful either. I forgot to ask how big the tank was, but I'm going to guess its 20-30 gallons based on the filter you're running. This should be fine for guppies and their tankmates.

Although there's no clear diagnostic information about what happened, I've found that almost anything disease related is mitigated with a UV filter. My suggestion is to try using one of these to soften the process of adaptation to your tanks. I use these, but since you have an external filter, you might prefer to use an in-line UV filter like this one. I've found that the first one works good enough for up to 30 gallons (although even 30 gallons is pushing it a little), but I haven't tried the second one and have no idea how well it works. I just randomly found it on Amazon and only link it as an example. You'd be wise to do some homework about whatever model you decide to get because they aren't cheap. (Protip: Remember to use fakespot before you buy anything on Amazon! Aquarium stuff is especially prone to lies and general fuckery from manufacturers).

The PROS:

• First, they don't cause any harm to plants, critters, or your biological filter.
  
  
• Second, they reduce the viral/bacterial/parasitic load on the tank as a whole. This is huge, because it lowers the threshold of immune system strength needed for critters to overcome pathogens. In other words, it gives weaker fish a better chance of adapting to the new pathogens by reducing their overall exposure to them in the water column. Furthermore, this increases the probability that your new guppies have a chance to spit out some babies. Babies are important because native-born fish have the highest chance of adapting to the unique ecology of your tanks. Its important to point out that UV filters don't (usually) totally eliminate pathogens, so all of the critters in the tank are still being exposed to the same pathogens, but the levels are low enough that their immune systems have a chance to mount a defense against those pathogens without the same risk of those pathogens taking hold.
  
  The CONS:
  
  
• Because they don't eliminate all of anything, they aren't good solutions when you need complete eradication. So, if you had ich in your tank (which must be completely eradicated), a UV filter will only work for as long as you use it. The moment you stop using it, ich will come back with a vengeance and kill everything if you don't intervene. So, they're best when you're fighting bacterial or viral diseases that are more opportunistic (i.e., they only threaten weak critters) versus pathogens that are invariably fatal, like ich.
  
  
• The other downside is that the bulbs burn out quickly (like, 3 to 12 months), so they can be expensive to maintain if you use them all the time. (NOTE: Some people choose to run them 100% of the time, but its definitely not necessary if your main concern is just getting your critters through the adaptation stage. If you're someone who keeps expensive/delicate/exotic critters like stingrays or arowana, perhaps its cheaper to run them constantly. That said, that's a whole different kind of fishkeeping that I don't know anything about. For peasants like myself, UV filters get the most value as training wheels for immune systems.)
  
  Here's what I did when I was in a similar situation. Perhaps you can have the same luck I did: A while back I ordered some endlers, introduced them to my community tank, and they all promptly died. My assumption was that the pathogens in my tanks kicked their asses, partly because they were ordered from a completely different area in the US. I decided to try again, but this time I was obviously very concerned about how well they would adapt to the pathogens endemic to my current tanks. So I started running a UV filter about a week before I planned to add them to my community tanks (A week seemed like enough time to drive the pathogen load down). I added the endlers to the tanks and kept the UV filters running until about two weeks after I saw the first endler babies born in each tank, so about a month after adding the endlers. Then, I removed the filters and crossed my fingers. Fortunately, the spike in pathogen load that surely came after removing the UV filters didn't seem to affect the endlers at all. As far as I'm able to tell, the UV filters gave them a chance to form a robust immune response. That was about a year ago and today my biggest problem is the booming population, but that's another post ;).
  
  EDIT; I should add that I would try a UV filter first partly because you'll need to start considering more drastic stuff at some point (e.g., chemicals) and since there's no easy way to know exactly what's killing the guppies, you'll probably need to use trial-and-error. As many of us learned long ago, the trial-and-error approach to treating disease in the aquarium is almost always a complete shit-show. Its expensive and in many cases you just end up harming the healthy inhabitants without actually solving your problem.

Well thats scary as hell. If people are euthanizing tanks with it it must be crazy hard to beat. I dont know much about it, but seriously good luck.


Last thing that MAY help, Im totally not sure though. They're worms right? Parasites? If so you can pick up [this]http://www.amazon.com/Bendazole-250mg-Fenbendazole-Powder-packets/dp/B009TAQ4OI/ref=sr_1_2?ie=UTF8&qid=1412396405&sr=8-2&keywords=fenbendazole) stuff. It works great on parasitical worms and free swimming. I used it for all sorts of random bugs I found in my shrimp tanks. They dont harm shrimp or snails, and obv. not fish. You can feed it to them through their food if its parasitical or dose in water column.

Happy cakeday! I've upgraded my filter to one that can handle up to 30 gallons (https://www.amazon.com/Marineland-Penguin-Power-Filter-70-Gallon/dp/B0009IODZG/ref=sr_1_3?keywords=fish%2Btank%2Bfilter&qid=1564378467&s=gateway&sr=8-3&th=1). So hoping I'm good on that front.

Do you think this would also be okay? https://www.amazon.co.uk/Superfish-Aqua-Flow-Filter-100/dp/B003ZGC22K?th=1&psc=1
(The 50 one)