Likely not, if it has a "magnetic" ballast typical of a old T12 fixture. I believe the strike voltage (point where gas ionizes and electric arc within the tube start the glow) for a T8 is higher than that of a T12. Magnetic ballasts for T12 won't reach the voltage level needed for a T8.
You can try it. If the ballast is insufficient it will either not light or flicker badly.
Depending on the length, you can still find T12 tubes of 5,000K to 6,700K in home improvement stores' lighting sections.
Eheim Pimp #459 - 2213, 2234
SunSun Pimp #47 - (Perfect) HW302