Originally Posted by AbsoluteRanchu

First of all try not to overfeed a weak tail fish, the bigger they are the worse the problem will be. Try to keep the fish relatively skinny. As GF ages, the tails and the fins will get thicker. This will ultimately help to cure some of the problem.

The problem with weak tail often comes from the way keepers keep fish when they are young. People tend to over feed in hoping that they will get a good size fish. You could try to mix more calcium for your fish along with the pellet food. It can harden the tail a bit, but it also have some undesireable effect as well, such as making the other fin parts harder and may cause the fish to start to swim very stiffly.

Try to be patience with your GF, make them grow steadily and never try to rush growth, it can only shorten the life of your fish. Grow slowly, grow strongly, I think.