Goldfish Smaller than Companion & Not Responding to Finrot Treatment
I have two feeder goldfish. They were purchased at the same time about a year and a half ago. They are currently in a 10 gallon tank with a 20 gallon filter.
One has grown significantly larger than the other, but the smaller over is still acting completely normal. The smaller one is now showing "shredded" fin ends, (still acting/feeding normally).
This fish had Finrot about a year ago & was successfully nursed back. It does not seem to be responding to the treatment after 2 weeks. Large fish is about 4" nose to fin, smaller one is about 2.5 inches long.
Because the smaller fish has had health issues in the past, I suspect it has always had an underlying health issue that the larger fish has been able to control through a stronger immune system.
The tail rot may be a secondary symptom of something else, like fungus can be for parasites.
There could be other reasons the smaller fish hasn't grown such as a physical problem like a deformed mouth that limits food intake.
Another reason could be if another fish species is also in the aquarium such as a bristle nose catfish. Many catfish and loach species develop a taste for the protein found in Goldfish body slime and attack them at night when the Goldfish are asleep and they are active.
If you can eliminate these reasons, then I suspect the fish has a health issue such as protozoan parasites or flukes.
You don't mention what treatment you administered for the tail rot, but apart from salt, most tail rot treatments don't treat parasites.
As the fish is not responding to treatment this time you will need a two prong approach.
1. Treat for parasites
2. Treat the secondary symptom (tail rot).
Clean out the aquarium of all plants, decorations and gravel. Make a complete water change (using a water conditioner such a Prime or Stress Coat). Clean the filter.
I am assuming that the other fish will be a carrier of any parasites so needs to be treated as well.
Put the fish into a salt bath of non-iodized cooking salt or aquarium salt at 0.3% solution (1 tablespoon per gallon, 3-4 grams per liter). Add 1/3rd immediately, 1/3rd in 8 hours, and the last 3rd 8 hours later.
Purchase a Praziquantel based medication such as API General Cure and follow the instructions on the container.
The Praziquantel will take care of any parasites, the salt will take care of any secondary bacterial or fungal issues.
Leave the fish is the treatments until obvious improvement is seen. Keep the water pristine with regular water changes. Get a water test kit so you know when to make water changes.