Okay, I just read the post about sick Nemo and was waiting for someone to answer. I just rescued my neighbors goldfish Wally tonight but not sure what's going on. He just came to his new 5gal tank yesterday with a friend from (yes, I know) that big store in town. Wally is a orange fantail goldfish about 1 1/2 inches long. He does have some tears in his dorsal fin and black streaks down to the base of the fin. I saw him when he came home and he also had some dark discoloration to his body but it was sort of hard to tell. He seems to be acting normal and is eating well. Today, my neighbor called to tell me that he was laying on the bottom of the tank most of the day and his body from behind his gills to almost is tail is really black, one side more than the other. I brought him home and put him in a one gallon hospital tank with just prime in it, an air stone, thermometer. We have well water and the levels are nitrate & nitrites are zero, gh 75, kh 40, ph 7.8, temp. 75F. at present and that I have just added maracyn & maracyn-2 per directions on box. The 5gal tank he was in last night, I only know that it was well water (the same) with prime, TLC, aqu. salt and Melafix with a whisper filter. When I got there the temp was 80F but the other goldfish looked fine and swimming around normally. So, anything else I can do? I'm somewhat assuming it's a bacterial infection, hence the antibiotics.

Its not bacterial infection. Its probably ammonia burns that are healing from being at the shop. and now he is in clean water the skin is going black with it healing. This takes around 2 weeks to get back to the normal gold.
You need to put him back in the 5 gallon as the the 1 gallon will pollute even quicker and make him worse.

The 5 gallon isnt really big enough either and if he is small you could get away with a ten gallon for a few months but ideally you are looking at a 20 gallon tank.

Ask your friend to get a test kit for ammonia as that is the main reason fish get sick without a filter.

On a 5 gllon tank you will be needing to do 100% water changes each day to keep him healthy.

Thank you Sandy! This morning Little Wally seems to be acting fine and swimming around. I'm still alittle concerned about the dorsal fin. Any suggestions as far as the tears and black streaks? I'll put him back in the 5 gal with his friend and check on him and the water. My neighbor is 91yrs old so I guess it's shared custody. I have a 30gal tank with goldfish and plan soon to get a 55gal for my 4 goldfish. I guess eventually I'll put her fish in my 30 gal. I haven't had any of these problems with mine, so I guess it's a learning experience. I'm still pretty new at this. Thanks again for your help and this great site!

Tears and rips could be from the other fish ripping at him maybe or the fish is stressed so lessening his immune system.
Keep up with the frequent water changes and adding dechlorinator and he should get stronger. The black should go in a couple of weeks and adding some melafix may help his fins get better.