Goldfish turning black, but 0ppm ammonia levels

hello! Rescued goldfish, i managed to get 3 of them and they are in a 20 gallon long planted aquarium. (i am trying to find another home with a better tank size currently) they are comet goldfish as far as i know and 2 of them are growing quickly and have no discoloration. One of them however has not grown since and has been turning black on its fins and underbelly. this was concerning so i checked all the water perimeters with my api test kit and there was no ammonia, and the water temperature is fine. there diet consists of a cycle of Saki-Hikari pellets, algae wafers, shrimp pellets, bloodworms and brine shrimp (frozen). i was wondering if anyone might have another cause, or if i should not be concerned at all. thank you! picture of the goldfish

 

Looks like normal colour change to me. It will probably keep changing as it ages. Wouldn’t worry about the differences in growth rate, either, as long as the fish looks and acts healthy. Some individuals just don’t grow as quickly as others. Genetics!

 

Thank you so much! What tank size should the goldfish be in? the tank right now

 

hi and welcome.

was that goldfish complete orange when you got him?

possible got ammonia burns from where you rescued him from. the black color is usually healing. make sure to test the water every week and do atleast 50% water changes every week.

from my experience you need 20g per goldfish. see my avatar, those are 12-13" long comets. in approx 90-95g of water. 75g tank with 29g sump.

 

He had a small amount of black on the tip of his fins, i know that he was purchased from a Petsmart or Petco, and was in a very crowded tank. This was at the school

 

i would say they were in the process of changing from the dark brown/olive fry color to orange.
check out these color changers
Goldfish Color change | Pond Fish Forum | 145357

just make sure you're doing weekly water changes.