So I was changing the water in the 10 gallon today, earlier in the week than I usually do since Tiger Lily had a minor tail nipping incident a while back. Anyway, as I was taking Tiger Lily out I noticed the guppy backed into a corner behind the filter with only half her tail:
What could have caused this, and what do I do about it? I do have some plastic plants in the tank, but they haven't caused any damage to my fish before, I've already tested them for bettas, and I didn't think they would affect her anyway. The filter could have gotten her, I guess- it's a power filter, and she tends to hang around the intake a lot. But she hasn't ever had a problem with it before. The only other thing I could think of is Sapphire suddenly attacking her, which I suppose is plausible considering the fact that Sapphire is typically quite aggressive... but I've never seen her try to full-on attack another fish (that isn't a betta), only push them away when they get too close. Sapphire has never had a problem with guppies before either, so I wouldn't have any idea why she would attack one now. The damage in her tail doesn't really seem like Sapphire could have made it anyway.
Here's some more info, if anybody can help me:
What size is your tank? 10 gallons, divided
What temperature is your tank? 82 degrees F
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? Female betta (Sapphire), as well as another female betta on the other side of the divider (Tiger Lily)
What type of food do you feed your betta fish? The guppy gets Tetra Tropical Flakes.
How often do you feed your betta fish? Once a day, in the morning
How often do you perform a water change? 3 times a week (sometimes more)
What percentage of the water do you change when you perform a water change? Usually, two 50% changes and one 100% per week.
What type of additives do you add to the water when you perform a water change? Just 2 1/2 to 5 ml of Stress Coat.
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed? Shredded tail. Duh.
How has your betta fish's behavior changed? She's being a bit more secluded and shy than usual.
When did you start noticing the symptoms? Today
Have you started treating your fish? If so, how? Not yet...
Does your fish have any history of being ill? No.
How old is your fish (approximately)? About 1 1/2 years old