I been on houzz for say 3 months i guess. You guys come up with great ideas. Thx

Do i need radiant floor heating? Keep in mind that its a sun room. A lot of glass = great heat loss.

You need a heat sink. A large mass of concrete that is insulated from below to retain heat. You need moisture management generally in the form of a central heating system to circulate the air.

As a heat sink i was thinking of using a water + solution (so the water can't possibly freeze). The problem is that i will need lots of water and how and where to store it

You don't need to re-invent the wheel. There are reasons standard practices become standard. The best method gets used, and the troublesome ones get forgotten. Water reflects more heat than a dark stone. It reflects the heat into the house.

AAn aquarium won't work unless you like poached gold fish :)

I want a sunroom, but i want it to pay off too

One law of thermodynamics is : A substance that heats up quickly shall cool down quickly, therefore, on the flipside, a substance that takes all day to heat up, shall take all night to cool down. That's why i was thinking of using (water + solution) instead of using concrete as the heat sink, i.e. concrete i think is going to liberate its energy too rapidly(cool down too rapidly)

In theory, water could be a method to store more heat, but I am not sure there is a good way to put water under a floor without ending up with an insulating air gap or a rotten floor. Even a waterbed ends up with air in it few weeks after it is purged of air. Unless the water were black, it would not absorb much heat as it is reflective and transparent, which is why it doesn't absorb heat from the sun very well. Water is actually a great conductor of heat which is why vertically drilled ground source heat pumps can be so efficient, and putting the ground loop in the bottom of a lake can be even better. Water is used in radiators because it does give off heat quickly. Water can be used in the interior of the structure, like in drums filled with water for table bases. But, it doesn't work well in the exterior envelope of the structure, including the floor. Here is a model floor that can maximize efficiency. You will need to copy and paste this into Google search because I can't seem to get a full URL for a PDF on my phone.
www.capital.edu/uploadedFiles/.../sunroom9821.pdf