Betta fish not moving much ; has 1 black eye

Question
Hello,
we got our first betta fish "big fish" a few months ago. he was the smallest and the palest in the store, in a little tank - so we decided to take him home and give him a nice big 5 gallon tank with plants, gravel and a big rock with a whole in the middle to swim through. We don't have a filter but change the water once a week (tab water with conditioner).
His color started changing to black, first the fins and then his whole body - but he seemed fine. And then the black color faded again and about 1 week ago his behavior changed. He just keeped sitting under his rock or on the gravel and he stopped eating (we fed him blood worms). He also lies kind of side-ways.

After some research I decided to put a heater in as I thought the water was getting too cold for him - it just started cooling down at night in the apartment a few weeks ago. he now has a heater in the tank for about 4 days and in increased the temperature gradually to 82 degrees. But he keeps sitting ON the heater! And he still doesn't move much and doesn't eat either. I also added pellets now to his diet since I read that bloodworms might be too much protein. Once in a while when we approach the tank he swims around frantically and then goes into hiding again.
In addition, I noticed last night that one of his eyes has turned black! I don't know anymore what to do to get him better. do you have any advice? thank you very much!

Answer
Hi Michaela,
This sounds like a bacterial infection which can be cured if caught early.  The Black eyes may be the beginning of Popeye.  When a fish stops eating, this is not a good sign.  Try to cure him with one of these medications...  Neomycin Sulphate, Penicillin or Tetracycline.  He could also have Swim Bladder Disease.  Did you notice if his tummy is swollen?  His tummy is just below his chin.  If so the medication mentioned should help him with this also.
Bettas should be fed a varied diet.  He should have betta pellets, plankton flakes, Daphnia is very important, Betta flakes, Brine shrimp, Glassworms, and Bloodworms should only be given to him as a treat.  Too many bloodworms are not good for his system.  He should also have one frozen cooked pea a week.  Remove the outer layer, cut the pea into little pieces, and feed the pieces to your betta one at a time, making sure he eats it, and that it does not fall to the bottom of the tank.  This is so he can clean out his system, and avoid Swim Bladder Disease to which the Betta is prone.  On the day you feed him the pea, feed him nothing else.  He should be fed 2-3 small meals a day.  Example: 2 pellets, or 2 flakes for one meal.  We must always remember that his tummy is the size of his eye.  Little meals, and not one big meal.
If he does not have a silk plant, it would be a good idea to get him one, so he can rest on the leaves.  Bettas love this.
When bringing up the temperature for bettas, this must be done very very slowly.  One degree every two days, as bringing up the temperature too quickly could be deadly.  I think you know this, but just wanted to make sure.
I hope the little man recovers.  Keep me posted on how he is doing.
Lynda