Ammonia Level? 0

Nitrite Level? 0

Nitrate level? 40 ppm

Ph Level, (If possible,KH and GH and chloramines)? 6.8

Ph Level out of the Tap?

Tank size (How many Gals) and How long has it been running? 46gal, 3 months

What is the name and size of the filter/s? Aquaclear 70

How often do you change the water and how much? 1/4 - 1/3 per 1 month

How many fish in the tank and their size? 1 x 4" Koi

What kind of water additives or conditioners? API Stress Coat

Any medications added to the tank? Fed him Jungle AB Med food for the last 4 days .. no change

Add any new fish to the tank? no

What do you feed your fish? Hikari Gold, medium

Any unusual findings on the fish such as "grains of salt",
bloody streaks, frayed fins or fungus? The only problem seems to be a small black mark on his right fin. It is flat, very black, and only about half a centimeter long. It's on the inside of the fin where he folds it against his boby to glide

Any unusual behavior like staying
at the bottom, not eating, ect..? He only stays at the bottom when he eats too much too fast. He has always been a very active fish and that doesn't seem to have changed. Im just concerned about what the black mark is. After testing, i noticed that the Nitrate levels are right at the limit of being gvery bad, so I'm going to begin water changes daily for a week to clear that up. Other than that, I'd just like to know if anyone things that it may be something else.

Hi, fish sometimes go through colorchanges so it might just be that. I thought that koi should only be kept in ponds because of their size, so Im not sure about keeping him in an aquarium. I dont think the black mark is anything serious.

Your nitrates are a bit on the high side,so it really would be a good idea to up the water changes and lower Nitrates to around 20 or under, before the fish does have any problems from long term nitrates.Doing weekly water changes will be more beneficial to the fish and the tank params.
As mentioned Koi do not really belong in an aquarium as they get pretty big and need so much more swimming room /space.
Normally,I would say that when you see signs of light black on fins and other places,to look into the water quality.However,I noticed that two of my fish that were Calico's at the beginning,and have gone through a number of color changes did indeed end up with very dark black markings on the edges of some of their fins ,to a point where the color does indeed look permanent.What kind of Koi is he?

The best i can come up with is that he's a Mukashi Ogon. He has a beautiful golden colour with patches along his body that reflect light like a gold nugget. Very burnished smooth look. His face is actually very similar to a japanese dragon look considering he's no dragon lol, but a fish. A very articulate mouth vs. most other Koi I have seen.

As far as tank space, I plan on upgrading as far as seems humane for an indoor Koi, then maybe an early retirement in my friends Koi pond

I worry about the black spot, but I've read that sometimes when there is a slight shedding in the slime coat there can be damage at that location and the black spot could merely be the healing process. I will definately up the water changes though so he makes to a ripe old age.

Your koi sounds beautiful,so you'll have to post us a pic. I guess time will only tell about the black being a healing sign from a problem.Like I said,my Calico's got a few black markings,and they have had them for many months with perfect water params.Black is not usually a color to just show up ,but I think in my case,that's exactly what happened to the fish.

Definately not easy to get a good pic of this guy lol. He's fast.

Gin Rin Ochiba might be a better breed assesment actually.