I am slightly confused. The fixture says 75W? But you put in 2x25W tubes. Is that the original W's on the tubes. <Standard T12 36" tubes are 30W.> I wonder if there is an incompatibility in the W's here.
It is quite possible that the 75W is an electronic ballast, in that case the tubes should work. So check the endcaps first before anything else.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.