You are not cleaning it properly, or at least often enough. Change at least half of the water at least once a week, but this is not the only effort in keeping things clean. You have let this get into a poor state, so, as soon as possible, remove the betta in the water that it is in, and empty the tank, bowl, etc. Now, if these things are too dirty, and therefore covered with bacteria, fungus; it is better to replace them, but, if they aren't too bad, clean them well; tap water is usually ok, but there are special cleaners for this that petco sells. Don't forget the inside of the bowl itself, especially the bottom. It will be covered with dung and rotting food if you have let things go. Petco sells a number of cleaning and scrubbing things for aquariums, but just buy a plastic covered sponge. It works perfectly, and is cheep. Now you have extra money to buy a new bottom- stone instead of glass, or a new plant or small interesting decoration.
Put everything back together, and fill it with room temp water. Then, and this is very important- add a few drops of water conditioner. Petco sells a good one called 'Splendid Betta Complete Water Conditioner'. This will get rid of the chems in tapwater that would sicken your betta. Now you are ready to go. Take your betta out of the dirtty water, and put it in to its' new clean and healthy home. Speaking of which, do not overfeed your fish. Feed them on a regular basis, of course, but not too much, or you will pollute the water, and have to start this all again.
I hope that we've been of help, and of course, feel free to ask any pet questions.
answered 6 months ago
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