Just an FYI, I'm not saying you have to worry about this but...
Evenly distributed load, like a basement sitting with 4' of water in it, isnt going to crack the floor. Concrete has an amazing compressive tolerance, it can handle lots and lots of weight being stacked on it evenly, evenly being the key to the whole equation.
However, 3 tons sitting in the middle of the floor not being evenly distributed across the entire floor is a different scenario. You've got a lot of weight trying to push the middle of the floor down, no weight trying to push the rest of the floor down... concrete does not have very good strength in that regard. I'm sure your floor is rebar reinforced so that will help.
Anyway, just my 2 cents. I've read posts in the large tank section where they had engineers check out their concrete floor in their basement and they didnt think it would handle the load long term so they had to go with different options. I'd be willing to bet in your case you'll be OK, because the area of the bottom of your pond is a lot greater than most people's large tanks which are not circular.
Do what you want, just want you to have the facts
I'd hate to see you get a nice new tank set up and end up with a cracked foundation in 6 months.
If I was setting up that large of a tank, I would at the very least just find out my basement slabs thickness and post on a housing forum or a engineer forum and tell them the dimensions of your tank, the planned weight and the thickness and age of your slab and see what they say. You could get actual good advice most likely for free, instead of advice from people who dont actually have any experience in the matter but are happy to chime in anyway