Angels fighting ... help!

Hi all

I have a 50g (4ft x 12" x 18") aquarium stocked with two angels who have grown up together in the tank over the last 18 months (bought 4 small angels but one died pretty quickly and the other one died about a year ago) + 15 tetra, 3 dwarf chiclids and 5 corys.

The two remaining angels have been OK with each other until a few months ago when one has started picking on the other.

Now the one being picked on has delevoped fin rot which I have been treating but obviously if they continue to fight, any treatment is only sticking a plaster on the problem (or at least I assume hence posting this message!)

Nothing in the aqurium set up has changed ... plenty of planting although I first noticed them fighting when I trimmed alot of the plants back as they were getting covered with brown algea. They both prefer the end furthest from the filter intake/outlet which I guess stands to reason.

I don't have space or a suitable tank to move one so can't separate them unless I give one away.

Would like to keep them both but obviously not if one is going to continue to attack the other.

Any thoughts on what I can do to keep them both in the same tank would be appreciated

Thanks for your thoughts.

I had a fight with some brown algae that appeared a little while ago which meant that the planting was reduced quite a bit. I have added new planting but overall it is still quite sparce. Oddly (to my mind) it is the aggresive one who is out in the open while the quieter one stays at one end.

I did wonder if I add one or two more would that work better as I had read that they will be less agressive in higher denisty but not sure if that is 'nice' for them