My Betta Is Sick!

I have had my Betta 'Snorkel' for about a month, and he has never been quite right. He would not eat pellets when I bought them, so I swapped to blood worms and have seen him eat one only twice. He is now having difficulty swimming to the surface, isn't eating, resting on his tail and as far as I know, he has constipation. I have tried feeding him a pea, but he won't eat anything at all. He is in a 20L tank with a heater and filter. His ammonia and PH levels are normal. HELP.
ps: When he doesn't eat, I remove the food so it does not affect the water.
Here are some photos of my poor guy and his tank.
Also, it is hard to tell whether he is fading in colour as he was a dullish colour when I bought him.
 

WildBetta, where are ye'?
 
Meanwhile, I'll give you my advice, but it is not from personal experience with bettas.
 
Bettas shouldn't eat veggies of any sort, as their digestive systems cannot process them.  Perhaps try to soak your food in garlic juice, then feed it.  This sometimes brings out the appetite of a fish.  Warning: I don't know if food soaked in garlic has any negative effects on Bettas, but this works for guppies when I want them to eat a pea.
 
Also, do you have any silk or live plants that reach closer to the surface?  If you can manage one that is may 1/2-3/4 of an inch from the surface, your betta could rest there rather than the floor.  This may help him stay closer to the surface.
 
Alas, that's all I've got right now.  I'll wait here with you till a "real" Betta expert comes along. 

First, do a water change.
Second, define "normal" pH and ammonia levels. Give us exact numbers please.
Third, add an air stone. I could be wrong, but this sounds like oxygen deprivation. The air stone will just help with gaseous exchange.

From what I see, your betta would benefit from more plants and a few more hiding places.

What is your water change schedule?

How did you cycle the tank?

He could also still be sick from being in the cup at the LFS. Perhaps he is suffering from long-term ammonia poisoning. WildBetta should be able to help more.

I went to the local aquarium yesterday, and they tested his water and said that everything was normal, so I cannot give exact numbers, I am sorry. They were extremely unhelpful and said "some fish just have internal problems" and there is nothing you can do. I did a water change last night, and he seems to be getting worse and worse. He is now lying on the bottom of his tank and his gills are opening and closing fast. I change his water once every week-2 weeks. I have a filter, which cycles the water. The aquarium said that this should give him enough oxygen. Do you still suggest getting an air stone? Will they sell this at an aquarium? I just really want him to be comfortable and not in pain. 
 
Thank you for your response!

By cycling the tank, I mean did you add ammonia daily, test the water daily, wait for the ammonia levels to go from 4 ppm to 0 ppm, and then wait for the nitrites to go from through the roof back to zero? I'm assuming not.
 
I recommend you get a liquid test kit immediately. My guess is that there is ammonia in the tank. The strips are not accurate at all. 
 
You should do another water change today. Do one daily with warm, clean, dechlorinated water (what do you use for this?) and you should see some changes soon. In a cycled tank, one should do a water change one or two times a week. 
 
I suggest you get an airstone and a pump (this will run you about 10 dollars). Make sure the pump comes with air tubing. These are really inexpensive investments. I always have a spare one in the house for this sort of thing. 


I'm going to suggest you have a read through http/www.fishforums.net/index.php?/topic/277264-beginners-resource-center/]this thread[/url] so that you can get some good information about fish-in and fish-less cycling (the latter in case this guy does not pull through).

Thank you both for your responses. Unfortunately, my little guy didn't make it. 

Oh no. I'm sorry to hear that. I do hope you continue to give betta-keeping a go. A well-kept betta tank can be one of the most delightful aquariums you can keep.