unknown fish disease

Question
Hello Lee, I have a 22 litre freshwater tank, I have had it for 6 months and keep loosing fish to the same problem but nobody seems to know what it is. I have 4 normal goldfish and 2 sucker fish, I have 1 fish at a time get sick, tail drops and the fish appears thin although it is still eating, it develops a bend in it's side just before the tail starts, over a few days the tail seems to break down and end up as just little strands like bits of cotton, this may be the bones of the tail with no flesh left? the fish always hides and dies after about 5 days. I keep my tank clean, change 1/3 of water every 10 days, I have a rock vac to clean rocks, I always use Aqua plus tap water conditioner when I change water and white spot remedy and have also tried Multi cure which treats white spot, velvet disease and fungal diseases all to no avail. I have even had the lady from our local pet shop come and have a look but she said she has never seen anything like it. PLEASE HELP as I am loosing hope of ever having a nice permanent tank of fish and also spending many dollars on treatment and new fish.....I hope you can help. Thanking you so much..Barbara

Answer
First and most importantly have your aquarium water tested for ammonia, nitrate, nitrite, and pH. Some fish retailers provide this service for free or may carry a testing kit you can buy and do the testing yourself. If you have goldfish, adding regular aquarium salt seems to really benefit them. Instead of the current conditioner you are using you may want to try using a product called aquasafe, it is a water conditioner and also a stress reliever, I have had great success with this product. Once you get your water tested and get the results please let me know so we can go from their with what could be wrong.