Buy a chemical test kit (like API's "Master Freshwater Test Kit") and monitor ammonia, nitrite, and nitrate. It sounds to me like your tank is not cycled yet, and so it sounds to me like you have way too many fish in an uncycled tank (depending upon how many fish we are talking when you say you have "some" of this or "a school" of that). We can help you a lot more if you are able to post readings for ammonia, nitrite, and nitrate. Avoid going the cheaper route and buying paper test strips; while it "seems" like that's a good idea, they are notoriously inaccurate, to the point of being entirely worthless. The liquid test kit will run you about $25-30, which seems like a lot, but will be probably the best investment into your hobby as an aquarist that you will ever make.
The chemical "Cycle" is garbage, don't waste your money. The only product that has a proven track record at helping accelerate your cycle is one called "Bio-Spira," which is stocked by very few pet stores since it must be kept refrigerated at all points in the distribution process. And even then success is far from 100%. Rather, the best way to help accelerate a cycle is to get some "filter gunk" out of the filter of someone who already has an established tank and put that gunk into your tank (especially in your own filter). Some pet stores will also give you a bag of such gunk for free; "mom & pop" aquarium speciality stores are much better at this than your large chain pet stores like Petco or Petsmart.
As others have pointed out, fancy guppies tend to be horrifically inbred (not by the pet stores, but by the large fish farms who raise these guppies by the tens of thousands in outdoor ponds and then sell them to pet stores all across the country). You can buy fancy guppies from 10 different pet stores in your area, the odds are all of them come from the same bulk fish farm and thus are interbred from the same genetic stock. Thus these guppies tend to be relatively fragile to begin with. Add poor water chemistry (*any* ammonia = poor water chemistry), that's an additional stress. Add pregnancy, that's an additional stress. Add other fish in the tank, that's an additional stress. Add the fact that some of those fish are "nippers" (most tetras) and others are pretty hyper (danios), that's an additional stress. Add (from the sounds of it) a relative lack of hiding places, that's an additional stress.
It's not all that surprising that your guppies mommas are dying.
If you want to see some successful broods, one thing you could do is this. Go to your
LFS and buy a small tank--even a small 2.5 gallon would work. Don't worry about substrate for it, nor a filter. Buy a tiny heater if your room temp isn't within a couple degrees of whatever temp you keep your main tank at. Fill the "birthing" tank (that's what we're setting up here) with a bunch of najas ("guppy grass"), maybe some java moss, perhaps a few stems of some fast-growing stem plants like anacharis, hornwort, etc. When you have pregnant females who look like they are close to giving birth, move them into the birthing tank. Since you don't have a filter, you'll have to do small partial water changes on the tank every few days or so (depending upon your plants and your light; if you have lots of fast-growing plants in there and are able to get the tank a fair amount of either sunlight or flourescent light, the plants may uptake almost all of the ammonia produced by the fish). Once the female gives birth, if she looks healthy you can move her back to the main tank; if she looks weak, keep her in the birthing tank for a day or two until she seems to recover. She will try to eat the babies, but if there is enough guppy grass and java moss, at least some will survive. It's best to set up this tank ASAP also, as the longer the plants are in there and growing (even if you don't have any female guppies in there), all sorts of tiny microorganisms and algae will start growing on your plants--this is great food for your newborn fry.
Once the fry have grown some, you can move them into your main tank.
And, after doing all this, your guppy females are still dying in childbirth, then you know it's not because of bad water conditions nor because the other fish are harassing them. At which point, if you really still want to have guppies, you might consider the (slightly more expensive) option of buying them online from an established guppy breeder. It will cost more, but in general you will get much healthier, stronger stock as a result.
Oh also, once the fry have grown a tad you will want to feed them with really finely ground up fish flakes. It wouldn't hurt to have a couple of snails in the tank to help clean up any uneaten food.
Good luck.
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