... I am surprised that is all that is dying in your tank to be honest!
Your tank is very much overstocked. I don't know if you have cycled your tank, or if you know what that means, but if you haven't you are going to have many more deaths on your hands. Read what I am about to post and then come back here and we will help you out ok?
For starters: cycling. In your tank there is a cycle that happens dealing with nitrogen. When the fish excretes wastes and as old food decomposes, these things turn into ammonia. This compound is very harmful to your fish even in small levels. In an uncycled tank this ammonia just accumulates and then everything in the tank would likely die. However a bacteria develops after a few weeks and it consumes the ammonia, converting it into nitrITE. this nitrITE is also quite poisonous to your fish, but just not quite as much. After a few weeks of the bacteria converting ammonia into nitrITE, a new one shows up that converts nitrITE into nitrATE. NitrATE is relatively harmless to your fish and can get to very high levels before it becomes lethal to fish, but not such a high level for invertebrates like shrimp and crayfish.
Your tnak specifically is very overstocked. The general rule of thumb for an aquarium for small community fish like you have is 1" of adult fish/gallon. Guppies are between 1 and 2 inches- we will say 1 here. Platies are the same. So right now you have over 20" of fish in our tank. That is well overstocked. On top of that you have 30 guppy fry that would grow up one day into adults too. As you can see, by taking the smallest value for your fish lengths you are overstocked- this was a very conservative estimate. Being overstocked means that the ammonia levels are going to rise to a point that the bacteria cannot keep up with converting it to nitrite, even after the tank has been cycled and established. Your fish will get poisoned if this is allowed to happen. They are also not going to get enough food to each of them because there are so many, and overfeeding can only make the whole problem much worse.
Unfortunately, I don't think you have much choice. If your tank is cycled already and you just didn't mention it, then that is good, but you will need to return a lot of fish... If you kept maybe 5 male platies and 5 male guppies and your corydora you would be ok. No females as they will produce babies like you wouldn't believe and then your problem is back again. If your tank is not cycled yet, you need to take all of your fish back, and then do some research on how to cycle your tank. Once it is cycled you can slowly add your fish to your tank.
Please ask us any more questions you have. I hope this post helped you out. Let us know what else you need and we will be happy to help.
Welcome to the tank by the way
