Hello; Are you by any chance in a long period without rain or on the other hand having a lot of rain? When soil begins to really dry out it can move enough to crack water pipes and shift basements.
If you have a lot of rain or do not have the gutters carring the water away from the house, it can cause cracks if too much is getting under the floor.
The weight of a 300 gallon might be over 3000 pounds which is not insignificant.
My guess is that nearly all cement floors have cracks, that the crack appears to be getting bigger should likely be checked. I had a crack in a basement wall when i moved in a couple of years ago. I found some microscope slide cover glass and epoxyed it to the wall over the crack. If the crack moves he thin glass will break. So far well over a year without breaking.