Danio Dead

Question
Dear Chris,
my tank is 5 gallon, I have 2 zebra danios (now only 1) , PH is 7,
ammonia is 0.5

I just start a new cycle for my tank 1.5 week ago, since my 5
tetras and 2 pearlscales died. I started with 2 danios, feed them
2 flakes per second day. I did 10% water changes for 2 times
since then.

Everything seems really good, they're very active. I tested the PH
and ammonia regularly, ammonia usually around 0.5. I thought
that's ok because it is a new cycle.
PH was quite acidic few days ago, around 6.6 so i put a pinch of
PH up and now PH is 7.

This morning, I found one of them died. I couldn't find the
reason because they seems to be really fine. Except that last
nite, i found that the smaller one keep chasing and nipping the
top fin of the other fish, which died this morning. I thought they
were just playing around chasing each other last nite.

Could the reason is bully fish? Or my PH up?
This is supposed to be a new beginning for me after my dying
fishes, and I thought I did everything right, so please help me
here.....

Thank you so much.  

Answer
Hi Pauline;

What a little devil! Sounds like you just have an aggressive danio. It's usually males that do that. Females have rounder bellies and tend to be larger. You might want to see if the fish store will let you trade him for another one. Just take him back and get two more that are the same size as each other.

It's also possible the dead one was more strongly affected by the pH rise. Ammonia is more toxic at a pH level of 7.0 and above. Or, it was just weak and would have died anyway. Get some plants and/or other decorations to give weaker fish a chance a rest from the other. In such a small tank they may not have room to get away far enough.

It is best not to bother changing the pH. Besides being dangerous when ammonia is elevated, it just causes a rollercoaster effect because it often changes back again within a few hours. Fluctuations like this are more harmful than letting the fish get used to a pH that we don't think is "right". A pH of 6.6 is really not bad anyway. Make water changes of 25% instead of 10% and it should remain more stable.

At Your Service;
Chris Robbins