Viettx, I'm really undecided. Flourish is one of the best fertilizers I know but I fear it contains not enough potassium to solve your present yellow leaves problem.
So likely you will need some more potassium at least for a while, may be you will need to add potassium regularly.
So, according to me, you may:
Buy flourish and add potassium buying salts
Buy fluorish and add potassium buying seachem potassium (expensive)
Buy another (cheaper) fertilizer a little more rich in potassium (like tetra)
As you don't have difficult plants may be other fertilizers are good enough.
Your question about fauna is really interesting. Your tank is very little so don't add fishes. Shrimps might be a good solution. Try asking the forum about the compatibility of shrimps with the fishes (and snails) you already have.
Madreefer, I'm sure my simple writing is due to my bad knowledge of English, but thank you anyway.
Tabs are useful if you have plants like cryptocoryne or echinodorus or other "rooted" plants.
Nevertheless, a lot of plants use aerial roots or take nutrition directly from elements dissolved into the water so these don't need tabs.
Furthermore, if you have a good soil, this ensures a good water circulation and even rooted plants may be "reached" by minerals dissolved into the water.
I, for myself, use tabs, but I have a big and heavily planted aquarium.