how to maintain the black color in a goldfish indoors?

[Death03]

Hi guys my first post here in AC. So here it goes, I currently have a black "thai" ranchu in a 40g with 3 other ranchus. When I first got him he's really black but with a golden belly. I only have him for like two weeks right now and all of it's color is starting to turn gold-ish.

I read online that to keep goldfish really black they have to be exposed to sunlight but I currently don't have the luxury to do that since I keep all my fishes in an aquarium, and when I'll be home after work there would be no sun for me to expose him to lol. Also the seller where I bought him to keeps all his goldfish in a pond so he would be exposed to sunlight daily.

I currently ordered a "full spectrum" led lights thinking that it would help him retain his black colors, since right now I just have "regular" led lights in the tank. Do you think that would help?

His current diet is pellets (saki-hikari balanced, hikari lionhead, hikari wheat germ), peas, and vegetables.

Here is a comparison of when I first got him and today. As you can see he's really darker before:


Any advice to keep/maintain the black color of a ranchu?

Thanks!

 

[FreshyFresh]

I had a really cool dark grey fancy goldfish a few years back and it eventually reverted back to his default orange. Wish I could help. I have no idea what it takes to keep them gray/black.