Goldfish Help

Hi! I’ve had my goldfish since May 2019 and I noticed he was a little down on Saturday April 11th. I got him a bigger tank, changed the water, added an air rock, etc. He’s unclenched his fins and has been swimming around more but I’ve noticed that his water is cloudy (despite rocks being rinsed and water being changed on Monday) and he’s not been eating. He has these new strings on the back of his tail that developed today... Any idea as to what it could be?

He appears to have fin rot. Do 75% water changes daily for 2 weeks. What size tank is he in and did you cycle it first? Are you aware of the aquarium nitrogen cycle?

it's excess mucous caused by poor water quality. do the daily water changes like deanasue said