Agreed with Miss Wiggle.
We definitely need to know what is in the tank and how big it is.
You say you had it set up for a year, everything was fine and then you put more fish in it and they started dying.
What was in it when you had it set up and what did you add that seemed to trigger this? You might have introduced a disease. What do the fish do before they die? Are there any signs of bloating or loss of weight? Do they swim eratically, become agressive, hide all the time, refuse to eat? Do they change colour, lose their scales, or develop spots, patches, holes or cuts? We really need to know things like this to help you.