What temp do keep them at? Tigers are quite prone to bacterial infections especially at warmer temps. Also, if they are only dying at night is it possible that O2 levels are dropping? If the tank is planted, at night plants use O2 and release CO2. Do you run an air stone or have good rippling at the surface? Just my initial thoughts. If it were me I would probably do a good sized water change and increase aeration. If they were still dying then I would medicate for bacterial infection.