CO2, what is it and how to test?


Reefing newb
I haven't been able to do a water change for quite a while but plan to do a big one this week. I have a small red planet acro in the tank that's always been a nice pink color but it's basically bleached white now and I read that some level of CO2 may be present but I'm not sure that a simple water change will help in getting it back to normal. Also, if levels are corrected over time, will the acro color come back?
How did it bleach? Top down, bottom up? If the skin is still there, yes it will color up. Co2 is not the issue here, high nutrients sound like the culprit. Flow and lighting also are factors. Please tell me a little more about the set up and you water stats.
Ok, finally was able to run the tests and came up with:

Ammonia - 0
Nitrate - 20
Nitrite - 0
Ph - 7.8
KH - 6 (107.4)
Phos - 0 (Always get this number)
Calcium -23 (460)

My Alk is a bit low, not sure if that would be the problem. Salinity is also but that will be fixed this weekend when I do a good sized water change. Nitrates are at 20 but I have had that level the whole time the tank has been running. How it bleached, I'm not sure. I think the whole thing just gradually went to a white color. When I got it it was some red and some green. It then attached to where I put it and it spread over the rock it's on and was looking good, the whole thing having a pink color. Now I can still see the polyps but it's basically white. Also not seeing much growth at all from any other corals in the tank.

It's a 180 with a 40 gallon sump. Right now just the one WP40, will probably get my second in a week or so. Skimmer is an ASM-G2. I run 3W LED's over the tank, 160 total. Only inhabitants in the tank are a blue/green chromis and a small blonde naso tang. Both of them are fine and been in the tank for about three months now.
Your salinity is low like mentioned. If you raise that to 1.026, your alk will fall in to place at the 8 to 9 range. There is a lot of issues I see with the numbers you have stated, and keeping high ended brand name corals need extensive care. Alk issues are usually tip down and pests are from bottom up. When the whole thing turns white, is usually a to powerful light. The lack of flow could be an issue that the coral cannot get the nutrients it needs and it's suffering or to much nutrients are setting on the coral. I honestly could use a picture to help more.
I do take pics often to track growth. Here's two. One when it looked normal then the last one I took. First pic is from June 22nd, the last one just a week ago, July 23rd. So this is a one month gap where it was fine until what it looks like now.


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I have that on a large pillar on the top of it. Ill measure the distance from the lights to it when I get home and post. I have the lights in I believe 11 hours a day.
Ok, I have 7" from my lights to the water surface then another 6" or so down to the top of that coral. I really don't think it's too high plus I'm seeing minimal growth in anything else I have in there. I think it's some parameter that's off and that's causing it. Just don't know what. Big water change tomorrow then I should be able to do the same every two weeks or so. Hopefully it gets back to normal over time.
Small frequent changes are much better. Any dramatic change is bad, even if it's reducing a bad parameter dramatically.
Well it was just a 35 gallon change which I don't think is too big for a 220 gallon capacity tank. Was just big to me cause I hadn't done one in quite a while. I did it on Sunday, I'm testing this evening to see if I have any changes.
Did my testing today and got better results.

Ammonia - 0
Nitrate - 20
Nitrite - 0
Ph - 8.0
KH - 8 (143.2)
Phos - 0
Calcium -26 (520)

Alk and salinity is back to normal and ph is up at a better level. I get my second wavemaker tomorrow so that'll help also once I get it in there and blowing around. Hopefully all this cures the problem. If it doesn't get color back soon, I'll lower it to see if it's just too much light for it although I doubt it. They aren't exactly top of the line led's although they are now dimmable so they are on full blast when they are on. I also cut my lighting time down an hour.
It was either something off in the water and after I did the change it corrected it or the addition of the second WP40 but whatever it was, the color is now starting to come back on it.