New 35 Galon Tank, Thas may be a "sick tank"?

Clean the tank good with hot water. Then set it all up fill it with water, start the filter and let it cycle for about 2 weeks or so, and your fish will be fine. Also get water test kit to make sure that your water parameters are suitable to put fish in. And the guys fish probably died because he didn't cycle and stock it properly and fish got an ammonia poisoning, that would be my guess.