How to keep the Water Structure Level

[drwho41963]

I know I could purchase products at the fish store that can help keep the Acidity, Alkalinity, and Nitrogen levels in check. My question is what are the correct levels, and what product would you recomment to use.
I have the following fish in the tank, Fancey Guppies, Sucker fish, and Angle Fish.
What other colorfull fish would recommend to put in this tank. Also what id the rule on how many fish to put in a tank?