In a cycled tank, having no nitrate is good, but very hard to acheive. In your tank, it means it is not cycled, because it is the by product of a cycle. All your water changes have saved the lives of your fish so far, except that guppy.
Tiger barbs are notorious fin nippers, especially in such low numbers. I have a feeling your guppies are not going to remain beautiful for long. If you got a proper school, it may help, but then you are getting very close to overstocked. I would seriously rehome the tiger barbs, they are not good in a peaceful community.
And rehome the shark, and the pleco. They will both be too big, and the shark will be aggressive with everything when it gets older.
Try to get an API master test kit, it will do everything you need more accurately.
20 gallon with a male betta, neons, glowlights, and red cherry shrimp. (work in progess) Recently added a few LIVE plants and driftwood, Woooohoooo!