There are real issues just waiting to happen due to the selection of fish you have. It may appear "fine" presently, but this is putting several of those fish under stress and eventually something will give. Also, as fish mature, they frequently develop their inherent behavioural traits. Which brings me to the shark. Red Tailed Shark
is much too large a fish and much too aggressive for a 10g. You can read more in our profile, click on the shaded name. This fish will be downright nasty and likely kill most of the other fish in time, not only because that is it's nature but in such a small space the added stress will worsen this. Tiger Barb
are naturally aggressive and as has been mentioned need a larger group, but in a larger tank. Please read the profile for more, I needn't repeat all that.
The glass tetra and Black Neon Tetra
are shoaling fish, they need to be in a group. Six is usually considered minimum. They are under severe stress without companions of their own species. This is how nature made them, we can't force them differently.
The 6 Zebra Danio
are in a decent group, but for such an active swimmer a 10g is not sufficient space. Moving these to a 30g will be better, but you still have the above issues.
Stress is a major health issue; sickness almost always occurs solely due to stress brought on by one of several factors. You cannot always see "stress" in fish, but it is still there.
On the nitrate issue, this is due to the high bioload as others have mentioned, or if nitrate is present in the source (tap) water as Mikaila subsequently noted.
Please read the profiles to better understand how nature is working here. We are all here to offer assistance as best we can.