@marquismirageI think you have a typo in your units, more likely it is kN/mē unless the tank is in free fall from a great height
There are two things to consider, the amount of weight the joist can hold and then secondly if the load is on legs how much load the floor itself has to withstand, hence the two figures.
On the positive side the 1000kg tank feels to be a bit of an overestimate, firstly the substrate and rocks displace water so its not the sum of the two, you can subtract the weight of their volume in water from the total. The sump will also not be full of water when running, perhaps only 30-50% will be water or you will have a flood when the tank syphons back. The tank and cabinet will be heavy but perhaps 100kg so maybe 700kg ish and that would probably be a bit of an over estimate still.
I AM NOT AN ARCHITECT OR STRUCTURAL ENGINEER and each house is different, so don't take this as a full answer but...
700kg is 6900Newton, 6.9kN and its spread over the footprint of the tank say 60 x 150cm which is 0.9mē giving 8.2kN/mē. This would be very bad if it were on one joist but you would put the tank 90° across the run of the joists so its weight is taken by probably 3 or 4 joists, 175-230kg per joist. This is still high but starting to sound more reasonable at least.
The other very important thing is where the load is on the joists length. If its in the middle this would increase the forces and again very bad but again the tank should be as close to the supported ends as possible so more of the weight is taken by the supporting wall.
The other point load figure - you have twelve legs so that is 58kg per leg, if the legs are small you can put them on blocks to spread the weight if needed.
So, you definitely still need to get qualified advice but, with care and proper calculation I think it is within the realm of possibilities.
I have my tank (in a bungalow) on a wooden floor and went through the same thought process. I have been under the floor and checked the joist span, their dimensions and where to put the tank so it is across the maximum number of joists and above a supporting wall. I then added additional 4 2"x4" vertical supports in front of the tank. No problems ....so far!