Aggressive fish

Hi,

I have three fantails in a 90 litre tank, I know that's a bit on the small
side, but I check the water regularly and the fish have been mostly OK -

except that one of the fantails has become quite aggressive towards the
other too. She (I am fairly sure it's a she) is quite a bit bigger than the
others (2" fish, the other two are 1.5") and is generally a big, fat, greedy
eater and active, etc. The others used to hold their own, but I've now
noticed both have splits in their fins and I've just had to take one out of
the tank into quarantine with red streaks in the tail fin surrounded by
whitish stuff. I assume this is some kind of bacterial infection. The big
fish was picking on this sick fish and chasing her around, fin nipping etc.
The quarantined fish is a lot better on her own. I am pretty sure these two
fish are both females, have seen what looks like expelled eggs, so don't
think this is a spawning-related incident, though they have been chasing
each other round, that sort of thing. I am treating and think since I've
caught it early the sick fish should recover OK.

OK, so I'd like some advice on long term what to do. Will the aggressive
fish always be like this? Is there anything I can do to stop her picking on
the others? Will I just have to put her in a separate tank? Will she be
lonely on her own - if so, how to find another fish which will not get
bullied?

Any advice appreciated.

Thanks, Alice.