I had the same issue with a tank parallel on the joist over a basement. I put two telescoping poles under the tank. Make sure you put the poles in place before you put the tank in place and make sure you bolt the poles to the top and put a piece or two of 2x8 or 2x6 on the bottom to distribute the weight on the concrete.
I had this setup for about three years, no issues with the floor bouncing or anything. I have since moved the tank to the basement, so I don't have them setup anymore.