Very interesting tank!!
What are the different species of fish you got in there? Discus look very happy! Their faces always look like they are smiling when they are young! I see you have some kind of hiding place for a fish in the back right corner. Whats it for?
I really like this tank! I can't wait to see how it fills in! As for ferts, I see you add Flourish tabs and Iron sticks. Other than that it is really hard to give advice especially on a 15g tank. What I would recommend is to watch the plants! If you see they are looking good and their color is good with no holes in the leaves or yellowing of leaves, or weak or brittle stems I wouldn't add a thing other than what you already do! If you start noticing anything I have mentioned or anything not normal then add accordingly.
In my 55g before I wend to a dirt substrate with a sand cap I dosed the EI method, but after "dirting" my tank I haven't had to add a thing in a while! I also have been battling Clado algae for a year now, but I think I about got it licked!
Anyway keep up the great work!