ok, if you can't get any water dechlorinator, you are going to have to boil the kettle full of water, tip it into a bucket to cool, and keep doing this until you have a bucketfull of water or as much as you need to change a third of the tank full of water. Make sure the water is cool before you put it in the tank, you can add a few ice cubes to it to help it cool faster (boiling and freezing tapwater helps to get rid of the chlorine that is in it- chlorine will harm your fish).
If your fish is swimming funny- on it's side and things, you need to start doing these water changes right now!!! Twice a day would be really good if you can do it- the more often you can change a third of the tank water, the better, even three times a day if you want.
Also, only feed him a tiny bit every second day- you can feed more later on when your tank is cycled, but at the moment, the extra food and poo in the tank is making the water more poisonous (more ammonia building up in your water).
Have you read about cycling yet? Also, later on your fish is going to need a much, much, much bigger tank- he will grow very big, and you need to get a really big tank as soon as you can, but in the meantime, keep researching and doing water changes.
Some gravel in the tank, and some live plants from your fish shop (they can show you how to plant into the gravel, it's very easy) will help a lot with your fishes health and happiness, and will help to keep the water cleaner.
so:
1. Water changes!!! (you can use a NEW measuring cup or scoop to scoop water into and out of his tank, then, when you go back to the fish shop, ask about a gravel vacuum and get them to show you how to work it)
2. research cycling
3. get some gravel and lots of live (aquatic) plants
4. research tank sizes for blackmoors
5. don't feed him much
6. get a big tank!
Hope this helps, keep asking questions and researching so you can keep him healthy and happy!!