fish tank cloudy, fish acting lethargic, worried she might die!?

i have a goldfish. i cleaned her tank about two days ago, i usually just give it a rinse a put fresh water in it, i've been doing this for the 15 years i've had her.

this morning she was floating on the top and looked like she was about to die. the water was cloudy so i cleaned it again, and put her back into it, but she began to look sick again! i think it could be a bacterial bloom, but i don't know much about that. also, i have a 2 year old nephew who likes to feed her, i'm wondering if he's overfed her while we weren't looking

i took her out of her tank and put her in the sink with one of the stones from her tank. i'm not sure what to do now. please help, i love my fish, and i don't want her to die!