I bought it second hand and was told its 35 gallons, but when I looked it up online it came out to be a little more than 30 gallons.

These are the measurements in inches:

L 36
W 12
H 16 1/2

30 gals is right. Tanks should be measured by inside the glass, it will give you actual gallons the tank will hold which with a 30 gal after adding substrate will be about 26 gal

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To find the NET Gallons use inside dimensions and measure the height to the top of the water line only (to the bottom side of the lip found on most aquarium frames). The formula is L x W x H x 0.0043 for rectangular aquariums.

Of course this will be less after addding gravel, decor and plants. So, I always add my water to my tanks by a set quantity of gallons and keep track, then write it down for future reference when adding any chemicals, fertilizers or medications, etc.