Belts and pulleys supplied with original RepRapPro kits (and I guess emaker ones) were MXL, which is an imperial size with a tooth pitch of 0.080", or 2.032mm. They are 1/4" wide, or 6.35mm. Belts for the Fisher were 2.7m long. See: [reprappro.com]
If your belt has a pitch of 2.5mm, it's probably metric T2.5 belt. Don't use GT2 belt with the MXL pulleys, it will throw out the steps per mm. Either get proper MXL belt, or replace pulleys and belt with GT2 versions.
GT2 2mm belt has a 2mm tooth pitch, but has a rounded tooth profile. MXL is very close to 2mm, at 2.032mm, but has a trapezoidal tooth profile. You shouldn't use the GT2 belt with the MXL pulley, and vice versa; you'll get backlash. If you got your Fisher from RepRapPro, get MXL belt. Get the ebay supplier to send you the right belt!