Looks great Davide!
I have a similar "grow room". It's really nice to just stick your head in there and fell all the light and heat..... especially when it's cold and rainy outside :)
What is the distance from the seedlings to the tubes? It's my experience that the distance should not be more than 5cm when you are dealing with normal lightning tubes.
Dan

Thanks Dan, The distance is about 20 cm!

what is the water heater all about

Very nice. How big is it? Two 18W tubes?

I don't think the distance matters much since you seem to have the whole thing surrounded by white insulation. That's my main suggestion: more white, less black. Black steals the light. I think you might need more light as the seedlings grow on.

Davide,
Very nice, I do alot of seeds starting year round and used to use the water heater method (now I use heat mats). I agree with Dan (astrophytumfreak), keep the lights closer when the seedlings start to sprout. One other suggestion is to put a layer of grit on top of the seeds before they sprout, helps keep humidity there (no problem with that for you) and helps support the little seedlings as they sprout. Try Mesa Gardens for named seeds. Here is a link to my seedlings of 6 months.
Dan

Here is a link that might be useful: My seedlings

That was very interesting. I'll have to build one or two of these!

Does it need a heater when kept inside at around 20 'C? (the tubes will provide heat, during daytime)

Thanks you all for your kind interest and suggestions... well:
James: I used a common aquarium heater immersed in a 1 Lit. plastic bottle (filled with water), this keeps the atmosphere and the layer of water at about 30°C.
SnB: The aquarium has a capacity for about 80 L. 2 tubes are necessary to cover and mime the whole spettrometric sun emission (blu + red zones). Regarding the black zone: The internal walls are all covered with plastic mirror exept for top (behind the tubes) As you pointed out I should add some extra mirror on the top.
Dan: You are right concerning the grit, it provides also some shady zones useful for germination.
I'll keep you upgraded....
Davide

Metamog. I think that if you keep the "thing" in your house the tubes will provide enough heat to reach 20-22 °C without any additional heating.
davide

Davide, my similar setup stays way above 20C (up to 30C depending on insulation and room temperature) with the lights on, but cools down to near room temperature when the lights are off for a few hours. I have the lights off during the evening when my house is warmest and use no other heating, so the temp sometimes drops to about 17C.

SnB, I think that 25-27 °C max temp. is ok for most of cacti but the problem is to cool down the min.temp. to about 15°C during the night cicle since, I guess, seeds germination takes benefit by considerable termal excursion.
davide

I'm not so sure about seeds needing the temperature variation. I know that some tropical seeds (not cactus) require constant warm temperature and freshly germinated seedlings will be killed by a night at 15C. I don't have enough experience with cactus seeds to know if any of them are like this. I've read that Gymnocalycium seed requires constant temperatures above 21C but it clearly isn't true and I've grown plenty with nights below that.