Hi friends first of all thanks a lot for your interest, i do really appreciate all the suggests and help you're giving me. If i don't take some of your suggestions it's not because i don't read/consider them, because i'm doing everything with a reason because i don't want to regret things in the future.
So...
Regarding the heating, my idea was to heat only the "big" tanks (150/200ltr) on the bottom and hopefully the room and therefore the other tanks will be heated too, this should be tested as soon as i put the first two tanks inside the new fishroom to test temperatures and shelving.
Regarding the ventilation, its true and that's why i'm preferring the curtain idea instead of the door with glass, but it that will result in a lot of heat loss i will have to make a door and keep it a bit open when needed.
About being efficient, it is one of my concernes but since it's a very small "room" i think that it's now worth investing in such heating and in a compressor, because the room is only: 9ft by 7ft by 7ft. Because of such a small room obviously i'll be having double shelving all around the room and the "extra" shelve on the floor will be used to store things such as extra equipment, fish foods etc...
About the painting... i discussed it a bit with some people and some told me that it isn't going to make a big difference between white and silver. Although i still believe that white should be a bit brighter, i think that if i leave it as it is i will prevent from having problems with paint fading / dirty wall or anything in the future...
About the shelving i re-considered using hallow sections and i'm almost there with the decision because i will probably decide when i start building the shelves. The new idea is, the bottom shelve will be over "knaten" / bricks so that it will be enough if i'll change from 200ltr to for example 600/700ltr... And the top shelve will probably hallow sections to save space for tanks and also i was told the bricks / "knaten" over wood isn't a good idea at all. Regarding space, the actual space i'll be loosing is a bit of space for storing equipment, which i could move unused equipment to somewhere else and not keep it in the fishroom to avoid space.
I think that's all for now, today i've emptied the room and gave it a very good clean and not it's spick and span, so hopefully this weekend i'll try to begin electricity 