Love it! Looks like you've got some cryptocoryne, rotala, and anubia plants. I can't identify the longer, stemmy one.
I would suggest getting a female or male (depending on the sex of the current fish) German Blue Ram, they're much happier in pairs.
What type of pleco do you have? Most pleco types get up to a foot or more in length.
Love the DIY Co2, let me know how it works out! Good lighting too! I have a 15 gal also, but I haven't found a light I'm overly fond of yet.
Also, with German Blue Rams, make sure the pH is soft, otherwise when you introduce the female, they may not pair off, and could be very unhappy. I've dealt with that a time or two with my rams, until I lowered the pH.
Best of luck! Keep up the good work!