Thanks hoppy - as far as I understand that's the same bulb with a different end cap fitting, which is just down to manufacturers preferences.
Could you indulge me while I summarize what I've learned to make sure I'm starting to get it? I'm finding this the toughest subject so far to get my head around!
All flourescent lights need a ballast to "start" and run the bulb, using different power levels to do so. Household CFL bulbs have the ballast built in, these ones have it in the aquarium lighting unit. Ballests can either be magnetic (cheaper, limited watt range) or electronic (runs a wider range of watts)
You can "overdrive" by putting a lower watt bulb in a higher watt ballast, but this reduces bulb life and might be dangerous. You might be able to "underdrive" by putting a higher watt bulb in a lower watt ballast. However, it's likely the ballast range means you won't be able to go up all that much higher than the ballast as it simply won't "start" it, or could flicker. Also, chances are it's not going to make any difference to the output anyway.
So for this unit - which I'm going to guess is magnetic, given that it's a pretty cheap beginner's tank, I "might" be able to run an 18watt bulb in the 11 watt ballast, but never a 36watt.
And if I want flexibility in plant choice I'm going to have to rethink the unit. Darn!
Given all that, would I be right in thinking branded aquarium lights are a bit of a waste of money for very low light/ display? I'm trying GE Biax 11w, 6,5000k, 900 lumen in there at the moment while it cycles to see if they have any adverse effects. (2 for £5 instead of £17 each!)
Final question, do I have to bear ballast in mind with T8s also?
I have a second fluval tank that takes 2 x 20w T8s. I'd planned for the aquastart to be my learning tank, but I'd like to sort out the fluval also.