Hi all. I noticed a couple of days ago that the oldest female swordtail (usually very peaceful) is developing a habit of nipping at my western rainbow. She pecks at the body near the gills 2-3 times before the rainbow chases her away, but she always comes back for more. There doesn't seem to be any evidence of wounds or bruises, but i've moved the swordtail to another tank temporarily to hopefully calm her down. Any ideas what might be going on? I don't think it's stress from over-crowding or bad water parameters. Territorial dispute?
Thanks for the help.

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55gal tank
2 Serpae Tetra
5 Pineapple Swordtail
1 Marigold Wag Swordtail
4 Peppered cories
4 False bandit cories
1 Oto
1 Bristlenose Pleco
3 Boesemani Rainbows
1 Western rainbow
2 angels

Hopefully the rainbow does not have a parasite.

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fw leiden since 1979, fo salt since 1979, mixed reef 55g 2002-2009. Strong emphasis on the tank taking care of itself. Balanced with plant life, no water changes, tap water, no filters in FW. Only dosing calcium, alk, mag in marine reef tanks. http://www.aquariumforum.com/f15/my-...ods-26410.html
recent tanks (till 2009) 7 years- 10g FW leiden 7 yrs, 55g mixed reef 7, 2 yrs, 20g FW leiden, 10 g fw leiden , 29g mixed reef, current tank 55g leiden