HEX and TET are chemical derivatives of Fluroescein. Each has it's own specific absorbance and emission wavelength characteristics. Depending on what equipment you are using to monitor the fluorescence, you should be able to distinguish between all them. If your equipment does not have dye specific filters or cannot be "dialed in" to a specific wavelength, then you would want to choose 6-FAM and HEX, which are further "apart" than 6-FAM and TET, or TET and HEX
6-FAM Max abs at 495nm; max ems at 520nm
TET Max abs at 522nm; max ems at 539nm
HEX Max abs at 538nm; max ems at 555nm
You can view a picture of the spectra for each dye on our website catalog. This will give you an idea of the spectral overlap for the dyes. Please see Dyes Catalog Page for more information.