hi there. i'm a fish beginner, and have a problem!

we have had two bubble-eye goldfish, one black, one gold. this last weekend, i added 3 little goldfish, the ones that i think people buy to feed other fish.

we wanted something to amuse a baby at dinnertime. :-)

the problem: the gold bubble-eye has a big chunk out of one of its back fins. it could have been eaten or torn off, though i can't guess where it could have torn. i can't find the fin or pieces, so perhaps it was eaten?

it's a 5-gallon tank with plastic plants etc (i know, the die-hards are probably groaning at me, but remember, we're beginners), but they don't seem very crowded to me?

did i do something wrong? do these guys eat each other after all? are feeder fish bred to be aggressive?

what can i do to make the torn-fin-fish okay?

You are correct in surmising that your tank is too small and that regular sorts of goldfish should not be kept with bubble-eyed goldfish which are simply too weak, deformed, and fragile. They cannot compete for food; they cannot get away from the faster fish; and yes, the regular goldfish will bite off chunks of fins and/or pop the bubble-eyes even if not on purpose. They are so much faster, they will bother the fancy goldfish in this case. The fins may have been ripped off when the bubble-eye rested and the smaller, "feeder" goldfish (probably comets) saw their flowing fins as something resembling moving worms to eat and tasted some. The bubble-eyes can't move fast enough to get away. Or, the fins of the fancy goldfish may have gotten injured on a filter intake or ornament. For these types of fish, you may have to put a protected covering over filter intakes like a piece of pantyhose or a mesh bag to keep the fins from getting sucked in. Experts recommend at least 5-10 gallons per goldfish so I suggest getting a few larger tanks and housing the feeders and bubble-eyes separately. They each have unique things about them that make them interested. The comets are fast and very active and always investigating things. The bubble-eyes are more laid back and stately. While they will stay pretty small (maybe 5" if you're lucky), the comets can easily grow over a foot long like some of mine. Smaller tanks do not prevent growth, they simply hasten death due to excess ammonia and crowding.

To treat the injured fins, I suggest adding a tablespoon of aquarium salt, some Stress-Coat (has aloe vera), and MelaFix by Aquarium Pharmaceuticals. None of these will harm any fish or good bacteria. If fungus or bacteria grow on the injured fins, then antibiotics and/or antifungals may be required.