Basic questions about a light

[senemedar]

Earlier today a lightbulb in my tank has finished it's life. I kind of expected it to happen, as it is pretty common for me, that something always break when I buy/get something new. But anyway...
Temporarily I used a lightbulb from my old tank, which is much smaller (I had to do some DIY and move the holders to a new places) and I noticed that my fishes has lost some of their colour. I reckon that might have something to do with the light temperature, right? But there's no note about that exactly, and both of the lightbulbs have the 'daylight' on them.
Another thing I noticed, is that searching online for the new light I found a T8 and T5 kinds, and I have no idea whatsoever what that's means-- I only know that I need a T8 since that is what it says on the broken tube. What does that mean and what is the difference between the two?
I also have got some live plants in my tank, but no idea what are they and how much light they need, so I don't know what wattage I need as well. The broken tube was 25W and was nice and bright, the one I'm using now is 18W.
Also looking for the lights I found a dark blue lights as well marked as 'night light'. Apart from it looking nice is there any benefit of having this kind of light?

And last thing-- I happened to have a spare lightbulb, but it keeps me bothered: how long can tank go without any light? I've read somewhere, that one only need the light for the live plants if there are any, but certainly there have to be something in it for the fishes...