Black mark under the Lower Caudal Lobe

Question
I just got my first aquarium and I have 3 - 1" Fantail and 4 guppies. One of the Fantails is starting to develop a black mark along the bottom part of the lower caudal lobe. Just in case one of the guppies I had to take it back to the pet shop since it developed a white spot on the right side of its body. The pet shop gave me a new one. After that i change the water and cleaned the gravel with a pump, plus added water conditioner and bacteria supplement. Should I worry about the mark or is it just a color change. By the way the fish is completely orange.

Answer
Hi Christian;

Fantails are a fancy type of goldfish. Sometimes black on goldfish can be from ammonia burns. Sometimes it's simply a natural color change. You didn't give me any information about your tank but if it's new, the black could indeed be from the break-in period that causes ammonia to build up and giving the fish burns. Not every fish gets them but ammonia is very toxic to the fish in other ways too. It burns the gills, eyes and skin and can cause illness and even death. Get a test kit to see where the ammonia level is right now and make water changes accordingly. Here is my own web page about new tanks to help you through this crazy break-in period;

http://www.xanga.com/Expert_Fish_Help

Goldfish are very messy guys that get very big too (6 inches) so they need ten gallons minimum per fish with good filtration. It's a common myth that they can live in bowls and small tanks. Each goldfish needs at least ten gallons to live a long healthy life of over 10 years. Here is a good web page about them and what they need;

http://www.firsttankguide.net/goldfish.php

Guppies are really not safe with your goldfish for very long. As your goldies grow, they will eat your guppies because guppies stay small.

I hope everything goes okay...

At Your Service;
Chris Robbins