I've moved your post to a new thread as it was not directly related to the other, and more may see it now which means better response. I'll start.
Not all manufacturers will provide some info like lumens, but in this case with T5HO dual tubes over a 55g I can tell you it is a lot of light intensity. I tried two 48-inch T5HO tubes last year over my 115g which is 5 feet in length and deeper and wider than your 55g (I'm assuming a 4-foot standard with 48-inch tubes) and it was far too much light, I took it back after a week and got a regular dual-tube T8 fixture.
You are correct, with T5 HO the watts/per/gallon idea is not accurate, as it basically is with regular fluorescent tubes, though the new models with less watts and equal intensity (energy saving and all that) has thrown some doubt into the wpg idea anyway even for T8 tubes.
There are a couple ways to handle this; if this is relatively new, the store may consider exchanging the fixture. Or perhaps the tubes, and you can get the T5 NO [=normal output, rather than HO high output]. Two tubes of NO will still be a lot of light, but more manageable.
The higher light intensity means you need more nutrients. CO2 may or may not come into it [more in a moment], but more liquid fertilization certainly will. Plants can only grow if everything they need is available, including light (adequate intensity and duration) and 17 nutrients of which carbon (CO2) is one, nitrogen (as ammonia/ammonium from the fish) is another, and then the mineral micro-nutrients usually supplied via liquid or substrate fertilizer(s). If the nutrients are not sufficient to balance the light, the plants can't use the light and algae will, big time. This is called the law of minimum, plant growth is limited by the factor in least supply.
Some of your plants, like Anubias
, like shade. Cabomba likes more light. If you arrange the plants accordingly, and have some floating plants, it can work well. I can provide more suggestions on tubes, etc. if you ask, but I hope the above will help a bit.