As you might know once I did a studio at home which cost me quite a fortune. It is sound proof. You probably saying what does sound has to do with temperature. I think they are quite the same as a principle to insulate. At this point its better to invest a bit and be satisfied tomorrow. You still can work with the things you gain(acquire). The best way to insulate a room is to isolate it. For example if you fix the walls with jablo and then do another wall made of gypsum, that would be great. That will take a bit of space from inside the room but, And maybe you would not be able to fit the aquariums you planned.
I liked the idea that you do the jablo under the tiles. that would make a difference. The wall stone and torba are conductors and should be kept in mind but the worst enemy is the air flow. If you close the room air tight you should be able to keep high temperatures with a very little power. But that as you might already saying comes with a problem. that of oxygen deficiency. So there must be some sort of ventilation for the fish and for your sake 
The fact that you built that wall of bricks and not solid stone, its very positive. Are the walls of bricks too? Most probably you are saying I m exaggerating. I say its according to what temperature you wish in the room? Are you satisfied with the present temperature? How much higher is the ideal for you? And you should consider the summer too.
But as I said before the more isolated and insulated the room the more you can change the temperature in the room with less power.
For example I heat my studio with only the PC in the winter, but to say the truth I have to open the window every now and then. Hope I be of help.