Welcome to the forum.
Sorry about the fish loss. Most illnesses/deaths are usually caused by poor water conditions, fish stress, etc.
How long has the tank been running? If the tank isn't fully cycled, ammonia and nitrite levels raise/drop frequently. Once the tank has cycled, these levels drop to 0.
How long have you had the fish? If the fish are new, then the increased bioload could have increased the ammonia and nitrite levels.
If the fish are new, how did you acclimate the fish? If the fish are not properly acclimated, the shock of the water temp, water parameters, etc. could have stressed to fish.
How often do you perform WC (Water changes) and/or gravel vacs. Performing water changes will help reduce the ammonia and nitrite levels.
Do you use a dechlorinator? Some water systems contain chlorine/chloramine. Chlorine can kill the nitrifying bacteria found in the tank, not to mention, is dangerous to fish. Using a dechlorinator (Prime is suggested) will help remove the chlorine. FYI, use a chlorine test kit to test your tap water to see if it contains chlorine. It may not be necessary to use a dechlorinator.
What kind of equipment are you running?
i have how do i know there is nothing on them that will reinfect any new fish added.can i some how steralize the plants?it seems so strange that all my platties, tetras and pleco died yet the only mollie i had survived and never seemed affected.tough little guy.
Ich is a parasite, which requires a living hosts. Ich cannot survive from a plant. If the molly does not show any signs of ich, then it does not have it. Ich's life cycle, is a couple of days.
I would wait about a week or so, before adding any more fish. Make sure you keep up with the maintenance of the tank.