I have a 10 x220 litre tanks and 2x 375 litre tanks, and a few more small ones on 100 year old wooden floor boards that are mounted on stump posts. The water alone weighs nealy 3000kg (6,600lbs) floor is sound and have had no problems. The weight is spread over the whole 11ft x 8ft (approx) room.
I think in most situations the weight issue is over thought to be honest, especially if the tank is near the wall which is the strongest part of the floor.
However you need to see how many joists are taking the weight and if the joists have enough support to transfer the weight down to the ground. If the weight is spead across a number of joists and the weight is distributed over a greater area you should be fine. If the floor flexes when filling the tank, that would signal that there isn't enough support. Tank placement in the strongest position is key.
We have one joist under the wall and 2 under the tank. The tank runs along with them. The are 2x8's 7"1/2. and there pine. I think the decking is 3/4.
Thanks for the reply's.