Why does my fantail fish keep floating upwards?

Hey there, Its probably its 'swim bladder' This is an organ that keeps it boyancy right. This organ can be affected by the fish eating to much and swallowing air in the process. And it could also just be because of poor quality food. If i was u id change the food asap. Go for a high quality fish food and never buy ur foods from a supermarket or discount store. Normally the more u pay the better the food. Here are some brands to look out for which are really good. Wardley, nutrafin and tetra brand. They are all really good but can be expensive at times but u should be able to feed ur fish less because high quality food fills them up more. If ur feeding ur fish flakes, keep to flakes just switch brands. If ur feeding them crumbles, stick to crumbles and so forth. And I just remembered that it could be a possiblity that there is too much ammonia and the water needs changing but because i dont know if uve cleaned it regularly i cant really say mate. I hope this helps :o)