I do occasionally feed then cabbage and peas and they seem to improve for a day or two but then they worsen again. I'll take your advise on soaking the food but how should i go about it? Should i just dip it in tap water or do i use a cup to store some tank water?The food you have is pretty low quality. I would suggest supplementing with leafy greens and peas. Also soak the food before feeding it so that the fish is not eating from the surface. Additionally, giving the fish an orange slice for about an hour once a week can help to keep colors darker.
The skin going transparent is not natural. If you can only do water changes once a week then I suggest closer to a 75% change on the weekend. Unless your tank is huge, a water change should take maybe a half hour tops. I would strongly suggest taking the time. If you absolutely don't have it, I would suggest finding a species of fish that does not produce as much waste and thus will only need a once a week water change. Goldfish are dirty, require a lot of maintenance, get very large, and live for many, many years.
I'm not sure about the orange so I might not do it. Doesnt it raise the acidity of the water?
Also no my tank is fairly small so i'll take your advise and change more.