Ive got a fantail gold fish, i noticed 3dys ago it has a red line on its tail and a red dot on one o

can you tell me wot it is ? all other fishes are fine.

Although the initial disease was probably tail/finrot, this sounds like it has now progressed to septicemia. Tail/finrot is characterized by fraying on the edges of the tail and/or fins and the usual cause is poor maintenance, stress, or a combination of the two. If not dealt with, it can become a bacterial infection called septicemia. Septicemia is noticed by the red lines. This is the infection going through the tail/fin into the fishes body itself (just like blood poisoning from an infected cut on a person that shows as red streaks from the cut heading towards the heart).

You fish needs to be treated with an anti-bacterial agent as soon as possible. Please do not use one of the low level tail/fin rot treatment for a cure as it has progressed beyond this point. You will need a quality antibiotic like marycin 2.

This looks like the starting sign of finrot.

If caught in the early stages this is not life threatening.

First you need to remove the cause.

Fish disease is caused by fish stress and poor water quality.

Water quality can be poor due to:

1) too many fish in the tank

2) overfeeding

3) insufficient oxygenation

4) insufficient filtration, build up of ammonia and nitrite

5) not enough water changes, build up of nitrate.

Get a test kit (if you haven't already) to check for ammonia, nitrite and nitrate; make sure your filter is adequate for the tank and regularly maintained; have a schedule of regular water changes; make sure that your fish have places to hide in; don't overfeed your fish.