for a 2.5g tank 14-25 watts should be enough. But then again it depends on what plants you want to keep in there. And how high you will have the lamp. Also another factor is how long you keep the light on. For smaller tanks its not necessary to have it on 10-12 hrs a day..
And im sure you are confused about the wattage of the bulbs. The Bulbs you have right now are probably 25w but they are equivalent to 100w incandescent bulbs. This means that in order to get the amount of light the 100w incandescent bulb gives off you would only need 25w of cfl (saving energy is the whole point)
it should provide more than enough light for the tank. I would try around 5hrs of light a day.. and see from there how everything goes, if you will need to increase or decrease the hrs..
yes co2 not necessary.. If you go with higher light plants then you can try co2. Make sure to provide good water movement to prevent any algae outbreak
The necessity of a filter really depends on your planned stocking, if you want any at all. In my opinion you could do well with just an air bubbler and a 25w light. Any high light plants wouldnt do so well in a 2.5 gallon tank....c'mon..just think about it, these plants usually get HUGE.
java moss is a hardy plant, and does well without Co2. If you want extra growth, then try some kind of dosing, like the stuff seachem makes.
I've set up a 2.5 gal recently (I guess kinda semi planted for now) with a red sea deco art nano for filter and a hydor mini (the flat one that goes under the substrate) for a heater and a 20watt undercabinet light strip (I also use sand as a substrate, will post pics of it later).
