The tetras would/will certainly snack on any shrimplets they can find, but I wouldn't imagine they would attack adult shrimp, so I am assuming it is adult deaths that you are seeing, yes? When inverts start dying while everything else lives, first suspect is always copper (or, less likely but still possible, any other heavy metals including iron).
Do your ferts contain copper? Are you using tap water that comes through copper piping in your house? Are you dosing high levels of iron as part of your fertilization scheme? Either of those metals could be the culprit here.
Another possibility would be your substrate, though all of the most common commercial ones are definitely shrimp-safe, but some of the more unusual ones might not be. (I've heard of one...can't remember the name...that is a black substrate that is essentially a finely ground up black slag from heavy metal mines!) Some homemade recipes for nutrient-loaded substrate "muds" also can contain copper or obscenely high levels of iron. Another possible vector for copper poisoning is previous chemical usage in the tank; a tank that was dosed with a commercial ick medication (most of which contain copper as an active ingredient), or with a snail-removing chemical (same), can be toxic to shrimp for 10+ years after the chemical was used. Basically once lethal levels of copper get into a tank, you essentially can never get it out.
Possibility #3, a contaminant (pesticide) that came in on one of your plants. Very low levels of pesticide residues can be fatal to shrimp, at levels where fish are entirely unaffected.
Possibility #4, stress-induced mortality due to the massive swings in water chemistry as a result of EI dosing combined with 50% water changes. I'm less knowledgeable here but in general, dwarf shrimp are much more sensitive (to everything and anything) than fish are. Unless you're in an emergency, 50% is probably too large of a PWC to do regularly on a shrimp tank. However, I realize it is necessary when doing EI fert dosing...which creates a problem. Not saying it's impossible, but in general, high-maintenance (read: highly fert dosed) planted tanks don't usually make the best tanks for dwarf shrimp. Shrimp are just really sensitive to water chemistry, and constantly dumping lots of chemicals in the tank can take its toll on them. Heck, even as something as nitrate poses a problem--most high light tanks need nitrate levels of, what, 30 ppm? Whereas the general rule for dwarf shrimp is to try to keep nitrates below 10 ppm. However, in most high-light tanks, if you have nitrate levels down below 10 ppm, you're going to have some massive problems with algae.
Anyway, those are a few of my ideas at least. See if your LFS has the API copper test kit, and if so, buy it and test for copper. Basically if you get any reading at all for copper, you're at lethal levels. That's still my #1 suspect and where I would suggest you begin your investigation.
