Goldfish being pecked at the back

[blackberry]

In my community tank of 20 gallons, I am finding one gold fish being continually pecked (near the tail) by almost all other fishes in the tank and this poor fish was trying to escape from the rest and was taking solace behind the filter and plants and with its fins closed most of the time.

Unsure of the issue, I separated it from the rest and put it into a separate 2 gallon tank and also added medication for suspected fungal infection/itching (sometimes this one seems to be scraping it's body against objects like the air pipe and plants).

My 'Aquarium for Dummies' book says, probably this poor fish has got worms in its stomach and the worms are trying to escape out of the back where the other fishes are trying to prey on them. I am worried if other fishes have already eaten the worms they too may have worms colony built up soon in their stomach.

Any clue to what may be happening?

Some background Info:
In the first week of getting this tank I got 2 pairs of gold fish, 1 pair of mollies, 1 pair of angels, 1 pair of kishing gouramies, 1 pair of blue gouramies.

In the 3rd week, brought a pair of moonlight gouramies, a pair of bigger guppies and a pair of pearl gouramies (in exchange of the blue gouramies which were always fighting).
The guppies died within 24 hours. Moonlight gouramies lasted for 2 weeks and then died, one kissing gourami died last week, her stomach looked yellowish, probably something wrong with diet. I have not found it eating much in the last 2-3 weeks.

One moonlight gouramies died due to bloated stomach and the other died suddenly after swimming swiftly across the length of the tank twice. Not sure if its due to any choking/heart failure ...
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[jinchi]

Blackberry, 1st, that seems like too many Fish. I count 16 or so, all together in a 20 gallon.

You did cycle this tank, correct? What are your water parameters? Ammonia, Nitrites, Nitrates, Ph, Temperature.

Get a bigger tank, or dont get anymore fish. Are you buying all the fish from the same provider? have you asked the provider for any advice?

 

[jsoong]

My 'Aquarium for Dummies' book says, probably this poor fish has got worms in its stomach and the worms are trying to escape out of the back where the other fishes are trying to prey on them. I am worried if other fishes have already eaten the worms they too may have worms colony built up soon in their stomach.

I doubt that is the case. Anchor worms might protrude out the skin, but they don't come from the stomach. Eggs from the worms are in the tank & the larvae swims around & attack other fish. If you have an anchor worm, a fish eating it won't get the worm ... rather, it is from the eggs in the water.

However, I don't think you need to go for esoteric parasites/diseases to explain what is happening. As others had said, you have a lot of fish in a small tank, all introduced in a short time. Chances are this is a water quality issue. Is the tank cycled? <And even if it is, adding all those fish at once will bring on a mini-cycle.>

Also, goldfish are not really compatible with tropicals. Having all those fish crowded together can certainly bring on aggression issues.

For a start, I would suggest testing your water parameters. In the absence of tests, do a large water change on spec (say 50%) every few days (or even daily if there are obvious sick fish) & see if things improve. And don't add any more fish till you sort this out.