Do you have a circulation fan yet? Is there a place outside the tank where you could try to grow them for a while to see if it’s somehow the tank itself?
Generally the only things that can kill a plant that fast are overwatering or something toxic in the environment. Well, completely drying out could do it for some of those plants, but then they would look shriveled and dry. I guess cold damage could too, but that presents as blackened leaves within about 24 hours.
Also, are these plants with strong root systems, or are they cuttings? "Weak" and "delicate" could suggest insufficient water uptake, which you would get with unrooted cuttings - they'll need to be kept in a pretty ideal humidity + air flow environment in order to prevent excessive transpiration and help them establish roots.