Weak koi on side

Question
Hi, my red koi fish (All red, 2 large front fins, 2 smaller back fins, Black spots on head and side) has been very weak yesterday and today.
Starting from yesterday its been on its side but a gentle poke will get it back to normal. We moved him to a seperate bigger bowl to try to nurse him back to health. We only had him for about 1 to 3 months at the most. The problem with the bigger tank is that theres a special kind of plant that only grows in water growing in the middle and the whole entire water has been changed a little less than a weak ago. He's been floating on its side to the top every while so we found an air pumper with air stones to put in the tank and it helped. He didn't float to the surface again. He also refuses to eat anything, the food I use to feed him were ones that floated to the top.
Today when I woke up he was lying on the side of the bowl not moving, so I used the fish net to poke him again and he swam for a little bit. I fear he's very weak. Is there any medication or food or some kind of easy treatement I can give him?

The tank is a really large antique chinese umm "bowl". It's too heavy to be moved. There is an air pumper inside with rocks on the bottom and a plant that grows in the middle. The bowl wasn't used for anything until then so we changed all the water less than a week ago.
My stepdad didn't get the fish at a petstore, he got it from one of those people on the street who sells fish. My mother refuses to buy a bigger tank and says there arn't any pet stores nearby. Im completely stuck. Please help!

Answer
Maysie,

Sounds like your fish has swim bladder. Goldfish and Koi are susceptible to this problem. In the bowl he is in, I would make it shallow, this gives relief to your fish as well. I would fast your Koi for 24 hours. The get a couple of frozen peas, thaw them out. Some people say to lightly steam it, but you don't have to. Then after you feed the peas, fast again for 24 hours. If he is not better repeat the fast, peas and fast. This should get him on the road to recovery. Some people also say that you can also give him a salt dip. 1 teaspoon of aquarium salt to 1 gallon of water. Place your fish in this water for 30 mins. Make sure you watch him while he is in this water so you can immediately remove him if he starts to have problems. You can do this 2 times a day for a few days along with the feeding of the pea. In order to try to keep swim bladder problems at bay, water quality is a must. Check your water perimeters at least once a week. I would also soak his food pellets in water before you feed it to him. This way the food will expand before he eats it. You can also feed him food that sinks. This way he won't run the risk of taking in air while eating. I would also fast your fish once a week. This tends to help too. I hope this helps.