Please help my Dojo :(

Question
Hi Renee,
My Husband and I set up a new 50 litre tank 2 weeks ago and last weekend we brought two fancy goldfish and a browny grey dojo. He was really tricky to catch in the shop but when we got them all home and put them in the tank everything was fine. On thursday we noticed that dojo had lost almost all his tail seemingly overnight and now there is only a few threads left. Also today he has developed lots of little red spots over his body and isnt swimming much :( is there anything we can do to help him? The two fancy goldfish are perfectly fine so we dont know what is causing it.
Please help!We havent had him long but are already really attached to him.
Kindest Regards
Jo

Answer
Hello Jo,

I will need more information before providing an expert answer.

Did you use a dechlorinator for the water?

What kind of filtration is on the aquarium?

How often are you changing water?  Did you do water changes in the past 2 weeks?

Are you using tap water or store bought water?

Thankyou and I will answer promptly once I receive your responses.

Renee