My first chicks at 1 week old. Need advice

I have had them for 5 days now and it seems that each day i lose one. They all seem fine and then one starts to look weak and lays down. Then after several hours it is dead. I have lost 4 now and one more seems to be going down. I can tell you that 3 looked that way when i got them and the other 2 had pasty butt when they came home and were a bit smaller than the others. The guy i got them from had hundreds in several brooders in a shed and i did notice a box in the corner with some dead ones in it. The setup seemed sufficient to me but they were way over crowded. Is there anything i should be looking out for? the 5 that seem good still are active and i have seen them all eat, drink, and scratch around. This is my first brood and i dont want to start out doing something wrong and end up losing all or most of them. I have them in the house at the moment in a large plastic tote with a red heat bulb, pine shavings i started yesterday, plenty of food, plenty of fresh water and the temp fluctuates between 91 and 96 directly under the lamp. It is around 72 in the room and the far end of the brooder is at 76. Any thoughts or ideas. I was going to get some medicated feed before the weekend. What about electrolytes. What kind and where do i get those? Thanks everyone.