Plants are not a great idea,the fish will pick at them,and make a mess.Stick with plastic.Look for conditioning food made by tetra.
OK,its an old tank so you should have algae in it?
Salt is added to help fish fight disease,i'm not sure what you used was it a liquid?
Conditioner is a most this puts the slime coating back on the fish which is lost durning transportation,stress,...Always use this when adding new fish,it also removes clorine from the water.
The lights you are using will heat the tank,so only leave them on when you want to see the fish.Your fish really like the dark.
Leave the bag in the water when you buy new fish for about 15 min.Then open it,put 1/4 of your water to the bagged water.Due this every10/15 min.until bag is full then let fish swim out.This way if you have a problem with your water condition the fish are introduced to it slowly and have a better chance of living.
Also add an air stone too your tank.This will give you more oxygen a the fish love the water movement.
You will never know why your fish passed,but it was nothing you did.I am just giving you precautions so maybe it won't happen again.
Never clean your tank....a fishes stomach is only as big as its eye...never let any food go to the bottom even if you have bottom feeders...always take a water sample with you when you by new fish,(test for ammonia,nitites,nitrates and p.h.)just to be safe.