dollarx2

i currently have a 10 gallon aquarium with 1 pirhana in it. i will soon be moving the pirhana to a 20 gallon before he reaches 6 inches. He is about 5 inches right now.
he has a big white blister colored and shaped thing to him. i have no idea what it is or could be. he acts normal and eats normal. i feed one 2inch guppie each day. About 2 weeks ago i didnt feed him for about 3 days because i was on vacation. when i cleaned his tank i realized he ate half of the plastic plant. but i think he just spit all of it back out because there was alot at the top of the tank. if you have any idea that somthing is wrong with him or know whats wrong with him a comment will be very appriciated.

COBettaCouple

http://www.klsnet.com/files/fishchart.htm - check this chart to see if you can match listed symptoms to what you are looking at on your pirhana.

i'll also move this to the diseases forum to hopefully get you more replies.

sirdarksol

So you're the one who stole the topic out from under my nose. (I wrote a reply and was told that the post was missing)

First of all, are you feeding the piranha feeder guppies? If you are, I would suggest that you find an alternative food source. Feeder fish are loaded with disease and low on nutrition (as their bodies waste away while trying to fight off the disease). If you can't afford to raise your own guppies, you might want to see if you can get the piranha to eat some other live food, such as worms. You can also try to supplement its diet with bits of raw fish dangled in front of it on a pair of forceps. Putting something like Vitachem in with the worms and/or fish prior to feeding might help, too. The extra nutrition might be necessary for your fish to recover, if it's facing some sort of infection.
Could he be omnivorous? Not all piranhas are carnivores, and the source for your LFS might not be the most reliable in the world. Lack of nutrition could be stressing your fish. (I mostly ask this because it went after something that looked like a plant)
I doubt the plastic plant did this to the fish, but I'm not sure.
Lastly, it seems that 2 inches of feeder fish is a lot of food for one piranha, but I'm not sure. I know that most carnivorous animals do better when there is some time between meals, especially meals that are just under half their size.
With this questions/comments on diet, I am not accusing you of anything. Rather, I think that fish stores don't tell owners enough about how to keep totally carnivorous fish (especially since they want you to buy feeder fish), so I want to make sure that you are aware of some things that they don't necessarily know/won't tell you. If you already know them, ignore the above comments.
Is the white spot fuzzy or not? Fuzzy could be a fungus.
Do the scales around the spot seem damaged? Are they falling off? Has the spot grown at all?
Have your pH, ammonia, nitrates, nitrites remained stable? This is again a possibility of stress, which can open a fish to infection.
Is there anything in the tank it could have injured itself on?