Unplug your heater if your worried. A steady drop in temp won't kill your fish, but it will make them prone to disease if it does get to low.
The danio problem you might want to make a seperate post as people will scan the heading of the post but will not see it's content if they chose not to look at it.
Include these things in the post.
a: how many fish have you got?
b: what types of fish and what types of danio are they?
c: how big are they?
d: water parametres?
e: how long the tank is running.
f: what are you feeding your fish.
and anything else you can think of.
With the size of the tank i'd say it wasn't a terrotorial issue, maybe a look into the male/female ratio might give you a good idea of what to expect.
I'd also like to suggest that I think danios are a schooling fish and need to be kept in groups of 7 or more to prevent one being singled out and harrassed.
also maybe adding some smaller faster "dither" fish would help.
Make another post and you'll get more response from more experienced people.
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