It's a tough decision. I personally prefer using Excel vs. DIY CO2 simply because I've found the latter to be more of a hassle than it's worth. On a somewhat larger tank, Excel will likely become more expensive in the long run, but you're not the first person to run into BBA issues with an underwhelming DIY CO2 setup. I very much doubt 2 bottles will be enough for a 55g, and IMHO, that is likely a big contributor to your problem.
A third option is to do neither. There are plenty of examples of low tech setups where no specific, external C source is provided. Whether your light regime allows for that, however, is uncertain. 2x T5HO over a 55g still seems like too much to me, unless you're going for a quicker growing tank...and if that's the case, you might as well invest in pressurized CO2, add some quicker growing plants, and reap the benefits.
A third option is to do neither. There are plenty of examples of low tech setups where no specific, external C source is provided. Whether your light regime allows for that, however, is uncertain. 2x T5HO over a 55g still seems like too much to me, unless you're going for a quicker growing tank...and if that's the case, you might as well invest in pressurized CO2, add some quicker growing plants, and reap the benefits.
