Disease Problems In New Tank

[dma]

I am trying to set up a new tank. I have bought a 60L tank and I have put sand in to make it look more realistic. I added in 2 zebra danios in order to condition the tank. I've put some hardy plants in and have been adding bacteria and have done water changes. Everything seemed ok until a couple of days ago. One zebra died and the plants seem to have dark brown 'sand like' bits on them and are starting to go yellow. Then today I had to kill the other zebra as its body was red and there was a white 'fluff' coming out of its body
My question is what can I do!?
Is it best to start all over again? Shall I treat? For what? If I put new fish in will they die? If I leave the tank without fish will the disease go away or lie dormant? What have I done wrong? I used RO water / minerals etc to set the tank up. I have a low pH and high nitrites but the tank is still conditioning.
Any advice greatly accepted!
Thanks,
dma.

 

[WILDER]

A cycling tank does take it toll on fish causing ill health.
The brown to yellow on the fish could be velvet to columnaris, so any sign of laboured breathing or flicking and rubbing against objects in the tank.
Shine a torch on the fish with tank lights turned out to see if the yellow glows up like gold dust.

If the body of the fish was red sounds like septicemia.
White fungus can grow on dead tissue.