I am a structural engineer and I would not take any direct advice saying it will be fine as it depends on plank spans and location of tank. Also wall construction below would be something to consider.
Wait so are you on the ground floor ( I know you said first but the flors wouldn't be concrete if there was a floor below ?)
Most flats and maisonettes have concrete floors as generally the spans can be greater, it is easier to install on large sites and sound regulations are easier to conform too due to the nature of the construction. Cost also plays a factor but timber and concrete actually works out pretty similar to each other.
On a note though assuming that the building has been built in the last 15 years then it is feasible that the floor should be able to support the tank but you have to get someone to do a structural survey unless you can find out yourself which way the floor spans, what size are the floor planks, what dead and live load the floor has been designed to support. Explain where you are placing, i.e. close to structural wall or to a partition wall (99% will also just be partition or nothing below) which will need to be factored.
Fill your bath get in it then get your mrs in with you... Does your floor break through..?
Lets work out why this doesn't work.
The floor in the flat again will 99% be the same throughout the building which means they have done dead load checks and know the weight of a bath that is full to capacity. Assuming that the bath is 5ft in length 2ft wide and 1ft deep (standard bath size) then you can see that is only supporting half the load that the tank will be holding in water without additional weight of the cabinet, external filter/sump.
On another note if you do get in the bath with your misses stand up as that will displace the minimum amount of water while adding maximum weight. Although the bath itself may not like that so stand apart rather than next to each other to spread the load. (Don't recommend this as the bath itself is not designed to support a full bath of water and two people standing up in it)
Now for some basic maths. Assuming that you weigh in at a tidy 11stone and your mrs weighs in around 9 stone giving a total weight of 20 stone or 280pounds, your bath is 10 cubic foot. One cubic foot of water is 62.5 pounds so the bath water weighs in at 625 pounds when full.
Your tank water is double that approximately which works out at 1250 pounds without the tank, cabinet or external filter/pump. The tank glass weighs in around 250 pounds, the cabinet weighs around 125 pounds. So that is totalling around 1625 pounds.
You, your mrs and the bath are only weighing in at 905 pounds. The tub itself weighs in around 30 pounds assuming that it is a standard modern bathtub. So your total is 935 pounds.
Between your bath and the tank there is a good 690pounds or another bath and a bit of water weight your floor may not be designed for.
