Ah, I see I missed an earlier post where you alluded to that being an issue. Well if that's the case then I guess buying water might be your best option at the moment...but given the tank size, I'd seriously consider thinking about purchasing an RO/DI unit for yourself if you are not going to be using tap.
One thing that was sort of touched on by gmh and Byron is C source. For a 55g tank, DIY CO2 really isn't going to be consistent and may be causing more harm than good. That setup is really only good for 20-30 gallons at the most, unless you're running a LOT of batches at once. I ran in BBA on my 30g for that exact reason...the DIY setup wasn't enough, and the algae LOVED the shifting CO2 levels. Excel is a good product, and may be enough to supplement the lower light plants' C requirements, but it's an entirely different mechanism. Personally, I'd discontinue the CO2 and only add Excel rather than the other way around.
What did you decide to do with the lighting? I'd also say that you likely have too much light for what you are trying to accomplish. 3x T5HO, correct? That's quite a bit of light, even for a slightly deeper tank, given that it looks like you're only keeping species with lower requirements. While cutting the photoperiod might help, I'd honestly just stop running at least one bulb altogether, however you're able to accomplish that with your fixture setup.
Definitely see what the GH/KH is like though...I'm assuming that, if your softener is functioning correctly, you'll see very little GH, but likely some measurable KH depending on what the water is like coming in. That may not be helping the plants as they do need Ca++ and Mg++, albeit in small quantities, but you don't seem to be dosing any sort of micro mix.
One other little thing to consider purchasing is a TDS meter if you're going to be getting RO/DI water from whichever source you choose. I got my EZ meter for maybe $20-25 online, and that way you can at least make sure that the water is "pure"...because GH/KH wont cover every thing.