Welcome to the forum!
Looks like you have multiple types of algae. Second and third photos down show BGA (cyanobacteria). Last shot looks like some sort of hair algae. Here is a link to a good site for algae ID.
Great... I had BGA before I had this hair algae stuff, and I was happy when the hair algae came. (Cause it actually LOOKS better than the BGA, BGA looks extremely nasty and not natural. Not that either looks good, but you get my point).
If you are growing plants you need nitrates they shouldn't be zero. Algae problems usually start with too much light and plants that are not growing at their optimum. If you are going to be injecting CO2 it is even more important for you to start adding comprehensive fertilizers. There is a Dosing Regimes sticky at the top of the Fertilizers and Water Parameters forum that you should read, it is really helpful and you will learn about the different options. For a 12 gallon tank you can use liquid fertilizers from Seachem or Pfertz but the cheapest route is to buy dry powders from someplace like EDITED. They are cheap and one $20-30 order will last a few years for a tank that size. The Flourish you are adding contains mostly micronutrients, you do need to add those but you also need to start adding the macro nutrients, (Nitrates, Phosphates, Potassium).
Thanks! Yeah I figured I'd start out with seachem ferts, but eventually move to DIY ferts etc. I figured it was just one less thing to worry about for now.
I suggest you stop worrying about your pH and ditch the chemical buffers. They make a mess of your water chemistry and can leave it unstable. If you are using RO water you shouldn't need them.
I would look at your rocks as a possible source of what is increasing the pH of your RO water.
See, the thing is, when I first started using RO/DI water, my pH was going crazy. Bad swings, and I actually lost a fish. I've read that when you use RO/DI water, you HAVE to have SOMETHING to buffer the water to prevent pH swings. Plus a KH of zero isn't good either. And equilibrium just raises the GH of the water. I've read a GH of zero isn't good either as it'll put the fish in osmotic stress. Oh, and it's not my rocks. I've tested them with pH down. Not a bubble in sight. They're just slate anyway. Like I said, I don't know what it could be. I'm pretty sure I didn't buy marine gravel, but that is the only thing I haven't tested. If I had to guess, I would say that there is a phosphate buffer system established in my tank (since my plants can't use all of the available phosphate), and it's causing my pH to be higher than it should be.
For the algae, manually remove as much as possible even it if means cutting off leaves, cut light intensity and possibly photoperiod (shoot for 8 hours max for now). Keep the tank and filter clean and increase water changes to help remove algae spoors. Maracyn will kill off the BGA but it is expensive and if you don't get the tank under control it will only come back.
Thanks for the advice. I've been trying to clean the plants off with a non-used toothbrush. I've even pulled them out twice and just rinsed them completely off with water. (I have well water, hence why I use RO/DI. We have an NaCl water softener, and lots of Na+ in our water.) I think I'll eventually end up doing what you said, cutting back leaves and stuff. The photoperiod is already 8 hours, so maybe I'll unscrew one of my CFLs till I get this under control. I have maracyn (it's not THAT expensive in the whole grand scheme of things.), but I haven't tried to use it to kill the BGA.
Since the algae isn't really killing anything ATM, I think I'm going to wait a bit on your advice until I can get the ferts to fix the problem. Once I get all of the macro nutrients, I'll do a VERY thorough cleaning (rinse everything, H2O2 dip maybe?, clean sponges in filter, vacuum the gravel very well, scrape all glass), then I'll do a 70% water change and dose everything afterward. I'm HOPING that with the correct nutrients (and a kick start from me), the plants will grow crazy and take care of the algae problem themselves. Do you think this could happen? In the meantime I'm going to continue to clean the tank twice or three times a week to try to keep the algae down. Do you think this is a good plan?
Ugh... just cleaned lots of stuff in the tank. My plants do not like being cleaned of algae. The fanwort is very very fragile. Hopefully it'll bounce back. I'm gunna try to keep it algae free just by cleaning it every day until my ferts come. We'll see how this goes. I'm also cutting back on light, both time and intensity. I have both seachem nitrogen and potassium coming in the mail. I'll probably go out and pick up phosphorus, but I'm not sure. For one, my fish budget is maxed for the month (yes, 4 days in and it's already maxed...), and like I said, I think I have a build up of phosphorus in the tank. Maybe I should get my water tested for it...