The old "watt/gallon" rough guidelines are based on T-8 or T-12 tubes I believe, so a compact tube will be putting out even more light for a given wattage. I don't know the extent "more" but its pretty clear that you are running at least at the equivalent of the old "2 watts/gallon" or higher, which means you are in the "high-tech approach" range rather than "low-light" approach to a planted tank
Actually, it depends on the exact type of "compact bulb", any more info or pictures would help
. Are they the regular screw in type?
Compact bulbs actually give you less usable light than standard florescent tubes, they just have the advantage of being smaller, so you can fit more of them into a smaller space to get more light. But they provide less light for a given wattage compared to regular tubes. Due to the shape of them a lot of light hits off parts of the bulb itself and is converted to heat (hence most compact lighting units have fans on them), it is harder (almost impossible) to get efficient refelectors for CF bulbs and the other thing is that they give a very uneven spread of light when used over larger tanks - the point right under the CF has far more light than the rest of the tank, and this needs to be considered when planting.
If you really do have high light, improving water circulation (10x the tanks volume per hour is recommended) and limiting lighting times (7-8 hours is plenty) are good steps to take as Waterdrop has advised. However, you dot necessarily need to worry too much about injecting CO2. There are other ways to get around the high lighting that are far easier IMO, and it kinda annoy me when people tout things like "you have over 2wpg you NEED to inject CO2".
Floating plants, for example, block out some of the light and are excellent for tackling algae because they are so fast growing - being able to access atmospheric CO2.
But 2 x 15w compact fluorescents over a 20gal really isnt that much, and I would count it as low-medium lighting at best (higher light directly under the bulbs) - I'm not even going to bother trying to estimate an average WPG rating without knowing more about the types of bulbs and the shape of the tank.
A non plant related thing that I think should be pointed out is that Siamese algae eaters should be kept in groups (3 will suffice) and get too big and active for a 20gal - so it would certainly be in the fishes interest to either return it to the store, or get another couple and upgrade the tank.
As for your algae - more info on the light would be useful, as well as the rest of the set up (filter, any fertilisers, lighting times and plant species).
With more info, we can advice on what plant species you should use and any upgrades or changes in lighting times you need to make to the tank so that it will be very easy to have lush algae-free plant growth.
Just like keeping fish, growing plants is extremely easy, provided you follow good advice or have experiance
.
