crowntail fighting fish

Question
Hi. Do crowntail fighting fish like massive tanks? Because when i got him from the pet store, he was in the tiniest tiniest bowl ive ever seen. im surprised he could swim, now i have him in a bigger tank than he was in, and he loved it at first, always swimming around and such. But now he always sits at the bottom of the tank (ive had him for just over a month) and always looks glum. my sisters fish is always active and exploring his tank. i have read in a few different places ont he internet that some people have one of these fish in like a 2 gallon tank! my tank is about 2 litres and square-shaped. he seems depressed and i dont want him to have an early death, please help! oh, and i put him next to my sisters tank every 4 days or so to stimulate him so he gets his fins and gills out etc (never in the same tank, two seperate tanks just next to eachother) thankyou!

Answer
Hi Madeleine,
Putting your Betta next to another Betta is not good for him.  It is stressing him.  Bettas love big tanks, and should be kept in tanks of 5 gallons or more.  It is a sin to see how some Bettas are kept.  Vendors give the wrong information, and bettas end up sick, and die.  The Betta is the most mistreated fish on the market.  
We have to figure out what is wrong with your Betta.  Have you changed his water?  Water changes should be made every week without fail.  If you have a filter, change 50% of the water, if not, change all the water, always using a good conditioner such as Stress Coat or AquaPlus.  Is his water too cold?  Bettas are tropical fish, and live in 80 degree water.  When the water is too cold, the Betta is cold, therefore will get sick.  If you do bring up is temperature, do this very very slowly.  One degree every two days.  Bringing up the temperature too fast, will cause his death.  What are you feeding him?  Bettas should have a varied diet.  He should be having Betta Pellets one day, Betta Flakes the next, Daphnia the next, and bloodworks once a week.  He should also fast one day a week, and on that day, feed him a cooked frozen pea.  Remove the outer layer, and  cut it into small pieces.  Feed the pieces to your Betta, one by one, making sure that the pieces don't fall to the bottom of the tank.  Feeding your Betta one pea a week prevents Swim Bladder Disease.  Swim Bladder is caused by poor diet, which leads to constipation, then Swim Bladder.  If caught in time this disease is curable.  Never overfeed your Betta.  His tummy is the size of his eye, so we must always remember this when feeding him.  Feed him 2-3 small meals a day.  Example:  2 pellets for one meal.
If your Betta's tummy is swollen, it is Swim Bladder Disease.  Please check this, as Bettas are prone to this disease.  Feed him the pea, and nothing else for 24 hours.  If he will not eat the pea, and does not waste, you will have to treat him.  "Kanamycin" is very good for swim bladder, but very hard to find.  "Tetracycline" or Furan 2 may also be used.  Never overdose.
Check all the things I have written.  It could also be ammonia in his water.  Ammonia is the number one killer of fish.  Do a nice water change.  
Write back to me, if you notice something different that I have not mentioned here.  I love bettas, breed them, and know what is good for them.
I hope he recovers quickly.
Lynda