https://courses.cit.cornell.edu/arch264/calculators/example8.1/
Fill it in and you'll get the answer.
The OP is missing a lot. E.g.,:
1) the span the beam has to cover
2) how many pieces of lumber
3) what direction you plan to set the lumber
4) what type of lumber
5) the dimensions of the tank
6) tank (glass, plywood, acrylic, etc?)
7) height of the cinder blocks
I'd recommend posting a picture, drawing, etc. of your plan, just to be sure everyone is on the same page. I've successfully kept glass tanks of 50-75 gallons on 2x4s (horizontally) with open spans underneath over top cinder blocks (open end facing up/down), so it's certainly feasible with proper wood sizing and placement.